I would guess its mainly down to the fact there are very few black people in these countries so they are seen as a bit of a novelty by the citizens of these countries. Or is it something totally different?
Pretty much. If they’ve barely had any contact with people of a different ethnicity then there will probably be more chance of some racism. That isn't to say that they're all racist or that a majority are racist but you'll probably find a larger minority of openly racist people than you would in a more multicultural society.
It's hardly confined to Eastern Europe though - you'll find it in plenty of societies, African countries can be rather openly racist for example, likewise South East Asian countries can be too - low level open racism is tolerated in Japan for example, they'll have Japanese only bars etc.. no "gaijin" allowed. Likewise places like Thailand will terms like "Farang", Hawaii has "hapas"... if you go to the Middle East you're generally an "infidel" though if you're a (white) Western European or American you'll only be one step below the local Arabs... muslims from the sub continent aren't treated too well ditto to other migrant workers... British Indians/Pakistanis can be fine, so long as it is clear they're holidaymakers or expat professionals from the West not migrant workers. Black people can get a very raw deal in Asian/the Middle East regardless - negative stereotypes perpetuate.
I guess that's another difference - if a black person is to experience racism in Eastern Europe then it is more likely going to be from some football fans or some weird looks from some uneducated scumbags in the street, it isn't generally at an official level or from businesses etc.. a western black or Indian tourist will be absolutely fine to enter a top night club/bar/hotel etc.. they know full well you're a western tourist with money etc.. I've been on holidays/stag dos with black and Indian friends in Eastern Europe and they've generally had an awesome time... in fact the only other black person we saw in the whole of one trip was the DJ in one of the best bars in the city. This was quite a few years ago - my mate had people crossing the road to shake his hand because they literally had never met a black guy and when we went to a club he had a lot of female attention.