Saying that a country like Romania is not ethnically homogeneous is absurd. Gypsies exist but in low numbers and stay in ghettos. Muslims only exist in some countries of the south of the Balkans.
Communism. It's pretty much same reason in whole Eastern Europe. Some countries managed to recover faster for various reasons whether it was more useful industry which was pretty much centrally planned in whole Soviet block from Moscow, so some countries where more industrially orientated and other remained pretty much agriculture only.
Second important reason is geographical location, especially presence of wealthy neighbors - compare Czechia with Germany and Austria as neighbors to Romania whose economically best performing neighbor is Hungary (which is economically worse off than Czechia itself).
My team has two people who came from Romania. My company's site here has many more. I know a bunch of them. Your statement is absurd. I've had more than one Romanian literally tell me he doesn't hang out with that other Romanian because of the ethnic differences between them.
(Not hostility, just a lack of common culture).