Why was Vladimir Ilyich Lenin so popular in Russia?

End of war promise and skillful organization

First we must address the issues of Russian Empire at the begging of 20th entry. Social and economic changes have long ago outgrown political institutions of that country. You had masses of recently freed serfs, but with little land ownership. You had growing industrial working class, also living in poverty and permeable to various socialist ideas. You had rising middle class of (university) educated people, but with little political power. You had various nationalities with little loyalty to Empire, dreaming their own ethnic states (Poles, Finns, Jews, even Ukrainians and various Central Asian ethnic states) . At the top of that you had autocratic Emperor surrounded with his entourage where real power in the country lay

It was clear that such situation would lead to unrest at first opportunity. And this happened in 1905, when bad economic situation, defeat in Russo-Japanese war and above all events surrounding Bloody Sunday undermined faith in the empire, and led to strikes, demonstrations and even open rebellion in some military units (especially navy). Anyway, as a revolution, events in 1905 were failure but did force the government to enact Russian Constitution of 1906.

Constitution was work of compromise: Emperor retained lot of power, but this was limited by newly established Duma (parliament). Some political liberties were given to citizens and the press. However, little was done to improve economic situation, except perhaps insurance for factory workers. Peasants practically got nothing. More moderate and affluent among "revolutionaries" were satisfied and ready to move political battle from streets to legislature and institutions. More radical (socialists and communists) were not, but they did not have strength at that time for complete revolution. Instead, radical left groups that would become Bolsheviks in few years later, started organizing themselves in a firm, underground structure with centralized hierarchy and harsh, almost military, discipline. They were preparing themselves for violent takeover of power at opportune moment.

That moment came when WW1 started. Russia did not fare well in that war. Huge losses, relatively incompetent leadership that used soldiers like pawns but still suffered defeats, general underequipment plus huge economic problems (i.e. hunger) on home front due to the lack of male workers (which were conscripted). It must me noted that average ethnically Russian soldier did not see this war as "patriotic" - Russia proper was not invaded, fighting was mostly limited to border regions of the Empire which were inhabited by non-Russian people that did not want to be in Russia anyway. Germans themselves did not show much inclination to let's say capture Moscow or St Petersburg (Petrograd). All of that fit well into Bolshevik's propaganda that this was not their (soldier's) war . Of course, the fact that Lenin collaborated with Germans, and that they actually paid and organized his return to Russia to undermine the empire was not known to public at this time. Idea that we should "stop war" even with concessions to Germans (that later became Treaty of Brest-Litovsk) was not outlandish to average Russian soldier at that time. This became increasingly important when Kerensky government after February Revolution continued with Russian participation in the war.

Luckily for the Bolsheviks, their cooperation with Germans was swept under the rug by German defeat in November of 1918. They didn't have to pay Brest-Litovsk agreed reparations, and control of territories was determined by force of arms as everywhere else in former Russian Empire. As for Lenin himself, he was also "lucky" to die relatively quickly after the success of revolution (in 1924). Possibility that his death was caused by failed assassination attempt by Fanny Kaplan increased aura of martyrdom. Since Fanny Kaplan was Jewish, in later years Stalinist propaganda portrayed her as a foreign agent, or Trotskyist, although she likely was more inclined to Socialist Revolutionary Party .

In any case, Lenin could not be blamed for later "excesses" of Socialism, but as "founding father" of USSR always had prominent role in iconography. He remained romanticized figure, always "so young" as his time was identified with early revolutionary hope and zeal. Dissatisfied people often discussed "what if" scenarios, i.e. what would have been if he stayed alive for few more years. In reality, we know now that Red Terror actually started when he was still alive, and under his direct command. It is unlikely he would be more merciful than Stalin. But, as things went as they went, he remained relatively "clean" figure for communists to create cult of personality that remains to this day.

This contains several questions. First, why was Lenin (more exactly, his party) popular in 1918-1922. The short answer is "because they distributed landowners land among peasants" (Only to seize it back after 15 years of dictatorship). They also stopped the unpopular war (by surrender) and declared the right of nations for self-determination (only to conquer most of them back within 2 years).

Second, why was he popular after his death, and until now. Because his successors who established a dictatorship in the country with total control and huge propaganda apparatus created a cult of Lenin. They, the ruling party, not the people, erected all those monuments. For several generations the population was intensively brainwashed (and the part of it which resisted brainwashing was physically exterminated). These rulers maintained the cult of Lenin which exists (to a smaller extent) even now.

In the independent Ukraine people destroyed all monuments to Lenin in a short period since 2014. But another revolution (of 2014) was necessary to make this possible.

You may compare this with the cult of Mao. His rule led to death of more people than Stalin and Hitler combined, and completely ruined the economy. Still his cult exists. Because the party created by Mao still rules in China.