I agree. I've written all kinds of genres trying to find my niche, as it were. Some people admire me for it, but I just think that when I find my 'niche' I'll stick to it...
I think it could be because a writer enjoys reading/writing that certain genre. Also, they may be more accustom to writing in that certain genre. I for one enjoy reading and writing horror, though I have recently been working on writing more Fantasy. I think the reason that most of the things I write are Horror is because most of the books I read are Horror.