The same reason people thin MLP when thinking of unicorns. A certain depiction became so popular that the original depiction became lost.
I always think of a book I read in my childhood that has nothing to do with that show.
My guess, though, is that the myth of fairies/faeries are commonly aimed towards the younger female generation. The lore was mangled and mutilated over time, and as such, people now think of faeries as tiny female creatures.
I still think mischievous imps. --- Pokemon SoulSilver FC: 1335 1407 6427 When Life gives you lemons, throw them back and demand cookies instead! -Official Sandslash of the PMD3 boards