I just realized this the other day...when my GBA Lineup consists of All First Party Nintendo titles (4 of which involve Mario: Kart, Golf, Superstar Saga, SMB3), and the only 3rd Party titles I owned were either launch games or cheap, I realize that I don't buy that much handheld software.
For me, I think it's the lack of cheaper titles. I know that discounted GBA Titles are rare for me to find, and I've picked up a lot of Sub $15 GameCube games that have diversified my library there, but my GBA library hasn't had that chance to grow, which has proven quite annoying.