For the people who really, consistently like them, I don't think there's a reason per se. Not any more of a reason than any fetish. You might think it's strange to mention fetishes, but I mean in the sense that how you develop an attraction to it is the same.
I just assume I like them because I grew up around my brother and sister wearing glasses and learned to think of it positively. Got no other opinions about it at all. I don't see a girl in glasses and think anything about it.