I'd imagine that it would be far easier to turn from the path laid before you by a non-creator god, than it is to turn from the path a god created your entire species to follow. In other words, it's a case of religious doctrine vs. the very nature of the species.
As you yourself pointed out, Drow were not always evil. I'm not sure what the typical alignment of drow was before Lloth's betrayal, but at worst it was most likely chaotic neutral. Beholder's, on the other hand, have always been evil, and were intended to be evil when they were first created.
In the real world, human beings are capable of having differing beliefs than those who raised them, but we're obviously incapable of experiencing emotions and using abilities that our bodies are physically incapable of - indeed, I doubt most people could even comprehend an emotion they could never experience, or an ability they lack ("seeing" in sonar, for example). For a beholder to be good, it would need to be a mutant... and in a race which detests even the slightest of differences, it would need to be one of the fastest learners and finest actors ever born to survive for very long.
So what exactly are the chances of an individual which is a mutant, an extremely fast learner, and an exceptionally fine actor? Infintesimally small... though not impossible. Perhaps one in every ten generations or so, in the entire world?