I'm not an expert on the history of the GDR, but there are several aspects which could explain the result:
The GDR "Volkskammerwahl" in 1990 is special, since it was the first free election since the Weimar Republic in most parts of the GDR.
Generally speaking, this should have served the 'new' parties better than the 'old' coupled to their western counterparts, as they were known as "Block-Paties" or "Block-Pipes", whistleing after the melody of the SED.
The former SED (now at this election PDS) and the parties with "sister parties" in Western Germany (here mainly CDU and SPD and after them FDP and the Greens) could have used their organizations.
Oh, yes. The "block parties" tied to the SED in times of the GDR almost immediatly flocked to their alleged western counterparts, CDU to CDU, LDPD to FDP to name the prominent. slipping under their coats for getting their support (money) as well as absolution from playing pipe with the SED in foremr times.
Curiosly only the eastern SPD, finally 'freed' from KPD/SED suppression kept itself apart from its western sister. With consequences I will show below.
In contrast to these powerful parties, the opposition groups could have lacked either the organization or the experience for an successful election campaign.
True, the money and the expeience of campaigning laid with the westernly accompanied parties - heavily. Of which this ...
Helmuth Kohl as the current western chancellor was one of the stronger proponents for unification, which could have helped the "Alliance for Germany" (the CDU and some minor allies in that election) to become the strongest party.
... was one of the results.
We could also explain the low result of the smaller opposition groups, perhaps they represented more the peace and environment movement (which was connected to the churches) and therefore only a small part of the population.
No, they represented the bigger part of the population even with their more ... westernly seeming agenda. They were heavily supported on the low levels of politics, the "round tables" all over the former GDR.
But, as said, they lacked organisation for a big election.
And later on, with the discussions about how the reunification should take place - the bigger part of all parties as well as the population knew this had to come - most of the 'normal' citizen refraind to ...
... the improvement of the economic situation. It is therefore understandable that 40% elected the same party into power, which had at the same time the power in the BRD.
... on which ofc Helmuth Kohl could and did "promise" the most to gain in the shortest of time.
But there is a point, taht needs to be mentioned, that played a major, the outcome heavily influencing part in these elections :
the publication of "Stasi-Akten", the records the Stasi held about almost every ever only slightly prominent citizen, even if only on a regional or local level. And especially, if there was mentioned even the possibility of this person being an "IM" (Informeller Mitarbeiter = informal (and
informing) employee).
Perhaps the most prominent politicians 'suffering' these publications 'just in time' :
Wolfgang Schnur, a conservative, but an opponent to Helmuth Kohl. After these publications his partys record was almost annihilated.
Ibrahim Böhme, prominent leader of the SPD of the GDR.
Couriosly, these puiblications were medially covered mainly on the Kohl-position opposing persons .......
IMHO the 'poor' performance of other than the big, western allied parties were in essence due to
- - lack of organisation and funding (the "Block-parties" had)
- - lack of experience in campaigning (turning on the 'lower instincts' of the voters, the western ally-parties had/have)
and