Why was the capital of Ukraine moved in 1934?

It's hard to answer this question with provision of sources. I studied history in the Ukraine, and I cannot remember that this question was payed more attention than just "the capital was moved in 1934".

As a theory I can suggest, that this definitely was not "accidental", this decision had substantial reasons, and it was related to the hype about the so-called "Ukrainian question" before WW2.

Before WW2, territories of the present-day Ukraine with people speaking similar languages* were divided between the USSR, Poland, Czech-Slovakia, Hungary, Romania. And with the accession to power in Germany of Adolf Hitler and his party, with their claims for Lebensraum at east, this "Ukrainian question" gained traction in the international affairs. There were numerous claims outside the USSR, that there had to be created "the Great Ukraine" (by taking its main part from the USSR) and so on. This was augmented by various terrorist activities of Ukrainian "nationalists", supported by Germany, not only in the USSR, but in Poland also. Long story short, the was a struggle going between various powers in Europe for territories of the present-day Ukraine. This struggle had a necessary ideological part: the German Nazis and their Ukrainian ideological brothers had claims, that the Ukraine was being occupied by Communist Russia, and the fact that the capital of the Soviet Ukraine was elsewhere, not in Kiev, the city which historically (since the second half of the 19th century, when all this Ukrainian nationalism started with support of Germany and Austria-Hungary) was considered the center of the Ukraine and without any doubt was definitely the most important city on the territories in question, that fact of the capital not being situated in Kiev was not in support of the Bolshevik's claims, that the true Ukraine was the Soviet Ukraine, because it even had a different capital from "the true" one.

There might have been a chance, that the transition of the capital in 1934 was related to the more significant control of the area by the Bolsheviks in 1934, in comparison to, for instance, 1919, (this is not a trivial question, because the position of the capital, especially in a such "big-government" state, as the USSR and its republics were, the capital situation has a direct relation on which side the government employees' sympathies, loyalty etc. would be and so on) but I don't think so.

It looks, that the main reason of the transition was the galvanization of "the Great Ukraine" theme in the West since 1933 and the Soviet desire to buttress their claim of "the true" Ukraine.

And in order to show some grounds for my opinion, I would like to give an excerpt from a Ukrainian history text-book of 2006, by O. Boyko, in my translation (the text-book itself, the excerpt's original should be searched by the words "Ініціатором рішучих дій у вирішенні українського питання напередодні", because the pages in the file have no appropriate numbering):

Germany became the spearhead of decisive moves in the solution of the Ukrainian question at the eve of WW2. After several months after the Nazi's accession to power, in March-May 1933 Rozenberg makes semi-official visits to Locarno and London, where during secret meetings with Italian and British politicians substantiates "a plan of division of Russia by breaking off the Ukraine from the Soviets". Then in June of 1933 at the international economic and financial conference in London a demand to pass the Ukraine to Germany for "more rational use of this abundant land" is made openly. This demand was in the memorandum, declared by the head of the German delegation Gugenberg. And though in response to a Soviet diplomatic note on this the German side claimed that the statements in the memorandum were personal ones of Gugenberg — this was just a diplomatic maneuver. Ukrainian goalposts was becoming more discrete in the Nazi's plans of foreign expansion. In 1936, while giving a speech in Nuremberg, Hitler declared, that if the Ukraine, Ural and Siberia were conquered, "every German housewife would felt how her life has become easier".

…

At March 10th, 1939, while giving a speech at the XVIII Party Congress, Stalin noted, that the unquenchable loud campaign in the West about "the Ukrainian question" has as its goal "to infuriate the Soviet Union against Germany, poison the atmosphere and provoke a conflict with Germany without any visible grounds for it". [This was said after the Soviet-German rapprochement in 1939 had started to gain traction] Then he warned: "Of course, it might be absolutely possible, that in Germany there are lunatics, who are dreaming to attach the elephant, that is the Soviet Ukraine, to a fly, that is to the so-called Carpathian Ukraine. And if indeed there are such idiots, be sure, that in our country will be enough strait jackets for such lunatics"

So, as one can see there was a struggle about which Ukraine was "true" and to "which one" the other parts should have been "attached". So the transition of the capital, I think, somehow additionally substantiated the claim that the real Ukraine was the Soviet one.

The original words of the citation, so that anyone could check the translation even in Google translate:

Ініціатором рішучих дій у вирішенні українського питання напередодні Другої світової війни стала Німеччина. Через декілька місяців після приходу фашистів до влади, у березні — травні 1933 р., Розенберг здійснює напівофіційні візити до Локарно і Лондона, де під час таємних нарад з італійськими та англійськими політичними діячами обґрунтовує «план поділу Росії шляхом відриву від Рад України». Уже в червні 1933 р. на міжнародній економічній і фінансовій конференції у Лондоні відкрито висувається вимога про передання гітлерівцям України «для раціональнішого використання цієї родючої території». Ця вимога міститься у меморандумі, проголошеному главою німецької делегації Гугенбергом. І хоча у відповідь на радянську ноту з цього приводу німецька сторона заявила, що зазначені в меморандумі твердження належать особисто Гугенбергу і не погоджені з урядом, — це був тільки дипломатичний маневр. Українські орієнтири стають дедалі чіткішими у фашистських планах зовнішньополітичної експансії. У 1936 p., виступаючи в Нюрнберзі на з´їзді нацистської партії, Гітлер заявив, що якби завоювати Україну, Урал і Сибір, то «кожна німецька господарка відчула б, наскільки її життя стало легшим».

…

10 березня 1939 року, виступаючи на XVIII з´їзді ВКП(б), Сталін зазначив, що невщухаюча галаслива кампанія на Заході навколо «українського питання» має на меті «розлютити Радянський Союз проти Німеччини, отруїти атмосферу і спровокувати конфлікт із Німеччиною без видимих на те підстав». Далі він застерігав: «Звичайно, цілком можливо, що в Німеччині є божевільні, які мріють приєднати слона, тобто Радянську Україну, до комашки, тобто до так званої Карпатської України. І якщо справді є такі навіжені, можна не сумніватися, що в нашій країні знайдеться необхідна кількість гамівних сорочок для таких божевільних».

* the present-day Ukraine with people speaking similar languages This is my personal opinion as a native. I don't consider the Ukrainian language in Easter Galicia, for instance, and the one near Kharkov as the same language. And near Donetsk, for example, no variant of Ukrainian has ever be spoken by any substantial part of the people. I know, that in today's depiction of the events in the Ukraine western media show this picture: the unified people of the Ukraine stand against Russian aggression, and that they always wanted to be "free" and all this BS. But in reality the present-day Ukraine is a post-colonial state with artificial borders (the great thank you to the Bolsheviks), where even many people in the Zakarpattia Oblast (let alone Donetsk) have strong sentiments against "the Ukrainians" from the Lvov Oblast (a Wiki article about the Rusyns , not to mention the Galician/Transcarpathian schism in emigration).

The Bolsheviks moved back the national capital from St Petersburg to Moscow in 1918. (They did this to differentiate themselves from the Tsarists. Likewise, moving the Ukrainian capital to Kharkov in 1918 was "revolutionary." It was also practical because using Kharkov, a workers' city as a base, made it easier to "pacify" the Cossack lands of the lower Don and Volga.

By 1934, the Bolsheviks were no longer the "revolutionaries," but rather the establishment. They had crushed the (agricultural) "kulaks" in 1932-1933, as related in e.g. "The Harvest of Sorrow." and established the ascendency of the urban proletariat. Then it was possible (and it made sense) to move "back" the Ukrainian capital to Kiev for the following reasons:

1. Kiev was a far older, more "traditional" and established city, having been around for about 1000 years, versus less than 300 for Kharkov (in 1934).
2. Kiev is closer to the center of the Ukraine, a better vantage point from which to govern the whole province. Kharkov is closer to the (north) eastern edge. More to the point, Kiev is the spiritual "heart" of the Ukraine for reason 1 above.
3. There was (in 1934) no immediate fear of a German attack on Kiev (or the former St. Petersburg, then Leningrad) as there was in 1918. There was a period of "amity" between Germany and the Soviet Union that started with the Treaty of Rapallo, and continued under Hitler (to the 1939 Pact). Certainly, the situation was much more stable in 1934 than in 1918, and Stalin hoped to deflect Germany "elsewhere."

Prior to the formation of the Soviet Union, Bolsheviks had established Kharkiv as the capital of the Ukrainian Soviet Socialist Republic in opposition to the Ukrainian People's Republic which had its capital in Kiev (the historic capital of Ukraine).

The Bolsheviks won.

Following the 1921 Treaty of Riga, the Soviet Union extended control over what would ultimately become the Ukrainian Soviet Socialist Republic and a founding member of the Soviet Union.

With control over Ukraine established, Stalin was able to return the capital to its historic location in Kiev in 1934 as part of the Soviet restructuring and industrialisation of Ukraine.