The people of Moldova speak Romanian. Up until Stalin stole Moldova while everyone was panicking about the whole "Hitler" thing, Moldova was a part of Romania. So why didn't Romania and Moldova reunite like East and West Germany did after the Soviet Union collapsed?
It's not like Russia could stop or would them, as Russia seemed to mostly just be interested in Transnistria. So why to Romania and Moldova remain separate to this very day? Was it an economic thing? I know there's a lot of resistance in South Korea to reunification with the North because North Korea is broke and would fuck up the South Korean economy if they were to ever reunify, and Moldova is the poorest country in Europe from what I understand. (If only because Transnistria doesn't count as a country.)
Firstly, this is my first comment on this sub and as a romanian I have a high interest in the Romania-Moldova union topic.I will try and tackle both the historical and the political sides of the question, but I also suggest posting this question on the r/Ask_Politics as your question is directly tied with international law and with the political climate of the zone.
I will start with a short review of Moldova's history which I consider relevant to your question with the mainly purpose of emphasizeing that in recent history and till today Moldova depended(and depends) greatly on Russia on all domains, not just financially.
As a beginning, it is very important to know that the present Republic of Moldova territory wise is a result of the annexation of Basarabia by the Soviet Union during World War II, 1940 to be more precise, and the separation of it's territory in 3 parts.The central part of Basarabia is combined with the already existent Moldovian Autonomous Soviet Socialist Republic (encompassing Transnistria and which was considered a part of Ukraine) organised by the Soviet Union as the Moldavian Socialist Sovietic Republic and is subordinated to the authorities of Moscow.The annexation has a special kind of legitimachy as it is a result of the Molotov-Ribbentrop pact, solidified by the Churchill-Stalin percentages agreement.
The years 1940 to 1953 are essential due to the mass deportations to the gulag which were specific for the sovietic communist regime.According to the studies of Rudolph Joseph Rummel somewhere around 230.000 persons(most of them romanian ethnics) were deported and around 60.000 died, either in the gulag or on the way there.These are some big numbers taking into account that the current population is around 3.500.000.This coupled with the sovietic colonization resulted in the decrease of romanian population ratio from 74% prior of WWII to 59% post WWII.
The facts mentioned above lead us to the first problem regarding a possible union between Romania and Moldova, namely that a lot of present day moldavians are russophiles because of their russian or part russian origins and thus are against the unionist movement.Moreover, a big part of the unionist supporters are young moldavians who in time migrate to Romania and lose interest regarding the union(telling this from firsthand experience).The best example of this problem is the election in 2016 of Igor Dodon(clearly a russophile and some go as far as saying that he is just a puppet for Moscow) who was against Maia Sandu(a declared europhile and unionist movement supporter).
For a change of perspective we will look over at the other side of the Prut river, in Romania.For one part of the romanian problem you hit the nail right on the head as a big part of the romanian population is against an union with Moldova for financial or economical reasons as this union would surely come different problems such as: a slowdown for the economy, difficulty regarding administrative and judicial integration, devalorization the national coin RON and so on.
The other part of the romanian sided problem consists of a combinations of more political factors.Romania and Moldova are not strong countries in any domain(military,economy,political,etc) and both depend highly or are highly influenced by either the EU or Russia.Furthermore, Moldova is a country with a delicate problem regarding the Pridnestrovian Moldavian Republic(Transnistria) which de facto controls some of the territories which are de jure Moldova's property.All the factors taken into account, Romania as a country doesn't have the necessary power or influence to unite with Moldova as the interests of more powerful countries exist in the area(mostly Russia) and a scenario where a second Crimeea situation would take place in Moldova is highly feared by Romania(and in my opinion by most of the EU members).
The unionist movement tried to circumvent this lack of power by claiming that the Molotov-Ribbentrop pact is not legitimate, thus reguesting that it would be repeled with retroactive effects(for the past) and ,as a effect, that Romania should go back to the 1938 borders.This is highly unrealistic as the majority of european states recognized the pact between Russia and Germany to be legitimate, if not at the moment of it's birth then after the creation of the Churchill-Stalin agreement, and a denial of the pact's legitimachy would be either unfavorable to them or would bring about too many problematic (possibly even conflictual) consequences.
Bibliography: Russia, the Near Abroad, and the West: Lessons from the Moldova-Transdniestria Conflict bt William H. Hill- I recommend reading this to better understand why and how Russia can(and probably would) intervene to stop a Romanian-Moldova union.
Moldovans: Romania, Russia, and the Politics of Culture by Charles King
EDIT: fixed a typo regarding the number of deported moldavians. u/Kochevnik81, thank you for pointing out the mistake, I can't imagine how dumb it must have sounded prior to the correction.