Who voted against German reunification, and why?

There never was a "reunification".

Officially it was an "entry of the five new federal states into the the scope (reach?) of the German Basic Law", although the treaty accomplishing that was called Einigungsvertrag ("treaty of unity" or "treaty of agreement"!). These terms and words were very carefully chosen. First, to not upset people like Russians and Poles, since West-Germany still had held some weak claims on lost territory in the East. Second, because it was constitutional law that in case of a reunification of the whole of Germany the Basic Law would have to be suspended and the whole population called upon to give itself a proper constitution. None of the elites wanted that the population would have any say in that. It was thought the provisional Basic Law had been good enough, why change it now? Just because the provisional constitution demanded it? Laughable.

East Germany joined the Federal Republic as the five Länder (states) of Brandenburg, Mecklenburg-Vorpommern, Saxony, Saxony-Anhalt and Thuringia.

At first it was seen as the most practical solution to start to form a confederation of two still separate Germanies. Then, perhaps gradually, a closer relationship might naturally evolve and come to a real re-unification. When the tides turned towards immediate merger some were concerned because of the speed at which it all was happening.

Note that nearly all East-German political movements and parties started as reform-oriented socialist. Even now-chancellor Merkel joined as a good communist the movement Democratic Awekening when it was still a socialist reform movement for a better, that is separate and socialist GDR.

Also note that almost all Eastern parties were practically steam-rolled by their Western sisters. The Western counterparts had experience, they had money and they had almost nothing in common with each other. But some were able to adapt to the new situation and then jumped on the bandwagon.

Some West-German politicians thought of a hastily unified Germany as more of an economic problem than a gift, notably some Social-Democrats like Oskar Lafontaine. While not opposed in principle they partly opposed the terms and conditions. He was right in that Kohl's estimates for the current situation were economically unsound and financially hazardous and the promised prospects for an economically golden future in the very near term for the Easterners a pipe dream.

Some West-German politicians in the Green party were actually left-wing instead of terribly conservative as of today and opposed in principle a new Greater Germany, fearing its increased power would lead back to the old ways of power politics, militarism and war mongering. They were mostly right but now this party is changed into one of the fiercest, hawkish clubs around. One example was later Foreign Minister Joschka Fischer:

he penned an article with the heading: "Der Wiedervereinigung die Schnauze verbieten!" (Shutting up the re-unification!)

Notably, some of those conservatives of the West-CDU who voted against this treaty gave some frightening reasons for their decision. Count Huyn for example listed several procedural and general shortcomings of the current form of affairs but his actual language revealed something else, more deeply rooted. While the Greens voiced concerns about stricter abortion laws he battled even the coming compromise as fundamentally too lax and that everything likewise what had developed in the GDR should be condemned wholesale within such a treaty, if there ever would be one necessary. Even more extraordinary, he declined to call East-Germany by its name and referred to those lands as "Middle-Germany", clearly indicating his wish to reclaim the German borders as they were in 1937. (Despite his direct denial of the 1937 borders claim in the following passage)

Die Herstellung der Einheit Deutschlands bedarf aber dieses Vertrages nicht. Im Gegenteil: Die Einheit kommt durch die Beitrittserkärung gemäß Artikel 23 Grundgesetz zustande. Zur Erstreckung der Geltungskraft des Grundgesetzes auf die Länder Mitteldeutschlands ist lediglich ein Inkraftsetzungsgesetz erforderlich. […] Unzulässig ist in dieser Form auch die Streichung des bisherigen Art. 23 GG. Denn unter den Deutschen, denen — wie das Grundgesetz formuliert — „mitzuwirken versagt" war, zählen nach ständiger Rechtsprechung des Bundesverfassungsgerichts ohne Frage beispielsweise auch die Schlesier. Auch wer — wie ich — nicht die Hoffnung gehabt hat, man könne die Grenzen von 1937 wiederherstellen und wer — wie ich — seit Jahrzehnten für eine deutsch-polnische Verständigung auf europäischer Grundlage eingetreten ist, kann nicht dafür eintreten, Menschen stillschweigend durch eine Pauschalabstimmung Grundrechte zu entziehen. Verfassungsrechtlich muß hier eine saubere Lösung angestrebt werden, und völkerrechtlich muß man in Verhandlungen mit Polen zu einer Lösung im europäischen Geiste mit durchlässigen Grenzen und der Möglichkeit der Gewährung des Rechtes auf Heimat ad personam sowie der Sicherung der Gruppenrechte für die Deutschen in den Ostgebieten und einer Regelung anstehender finanzieller Fra- gen kommen. Dies ist allerdings politisch schwer möglich, wenn man einseitig auf Rechte verzichtet, bevor man das, was man erreichen wi ll, auch nur begonnen hat auszuhandeln.
__Plenarprotokoll, p17936
__(Technically he says despite he himself not having had "hope to restore the borders", others do have these hopes and Silesians – or other 'real' Eastern Germans – were now declined their rights to rejoin Germany, if they were still in Silesia, and denied their right to resettle Silesia if they were expelled after '45 and are now Bavarians etc. His use of words, among them "hope", make the rhetoric transparent that he champions all of that. [Silesians and Sudetendeutsche Heimatvertriebene were an extremely influential and important group of voters, almost exclusively for the Union parties.])

As already stated, the Socialists in the GDR had purged their Leninists. But that still meant they were now reform oriented socialists. They feared that all their socialist accomplishments they fought for and had built up in 40 years were to be now disposable in a capitalist Germany. They were right in almost every aspect and had to pay the price of being a Cassandra (as well as for the misdeeds the SED had heaped together) at the elections.

The new leader of the PDS, Gregor Gysi, famously concluded after the final voting results on this were read :

"Frau Präsidentin! Das Parlament hat soeben nicht mehr und nicht weniger als den Untergang der Deutschen Demokratischen Republik zum 3. Oktober 1990 (Jubelnder Beifall bei der CDU/DA, der DSU, teilweise bei der SPD) beschlossen. Ich bedaure, dass die Beschlussfassung im Hauruckverfahren über einen Änderungsantrag geschehen ist und keine würdige Form ohne Wahlkampftaktik gefunden hat."
"Madame President! Parliament has just done nothing more but nothing less than to determine the downfall of the German Democratic Republic to be on October 3, 1990. (cheering applause from the CDU and DA, the DSU, partially from the SPD) I commiserate that this decision was railroaded through parliament via an amendment and hasn't found a dignified form [of debate & procedure] without election campaign tactics."

The Eastern Greens were what had formed one of the core elements of the GDR opposition. That opposition started as movement to reform the GDR into a more perfect socialism., respecting freedom of speech and protecting the environment. They not only opposed capitalism as well as leninism. They also feared that one Germany would disrespect al the good aspects they saw in the GDR as it had been but also all the good aspects as they envisioned the GDR would be capable of becoming. Since they were quickly marginalised in the East and in their own unified party.

Lastly, when the time came to vote, it was also a democratic staement to vote "No" even if you would have been in favour. The last elections of the Volkskammer were such a clear statement of the voting populace to get Deutschmarks as Helmut Kohl promised and that as soon as possible that the possibility of success for a "no" was almost impossible.

Some important accounts of the events, contemporary and retrospective:
Die Nacht, als die DDR unterging
Historische Debatten (11): Weg zur deutschen Einheit
Wer die Qual hat, hat die Wahl
Ein Staatenbund? Ein Bundesstaat?
Wiedervereinigung oder Anschluss?

The exact minutes of the final debate for the West-German side (warning: interesting but lengthy read ;) and a list of who is for and who against (p 17896; no typo) is here:
Plenarprotokoll 11/226 Deutscher Bundestag Stenographischer Bericht 226. Sitzung, Bonn, Donnerstag, den 20. September 1990

The shorter corresponding list of East-German Volkskammer delegates and their votes is here.