I think it's a combination of low prices and the fact that (in their photos at least-which may be of somebody else) , they usually look attractive. However, on the rare occasions I have punted with one (usually by mistake), i have got a strong impression that their heart wasn't in the job, and they were trying to get as much money for as little effort as possible. I have had the uneasy feeling that they were lured to Britain from rural Romania on false pretences, and somebody has their passports for safe keeping.
I have come across one lovely Romanian, however. She spoke excellent English, and came from Timisoara in the disputed territory of Kosovo. She detested the Roma gypsies (who are most of the sex workers) and regarded then as a bunch of lazy, thieving, ill-educated good-for-nothings. Her comment "why are your police so nice to them"? sums up her attitude. And probably the attitude of most non-Roma Romanians.