Based mostly on the fact that the first introduction to technological shapeshifting I had (Animorphs) had much bigger clothing issues than Ben. I mean, most of Ben's forms take on his clothes, but Ripjaws, Wildmutt, Wildvine, and Heatblast are notable exceptions (and they seem to change "under" his clothes, with the clothing just somehow disappearing and then reappearing later). In Animorphs, it took them some time to learn to morph into clothes: their first attempts were rather clumsy, with the first few times they turned up totally naked, then they figured out leotards and form-fitting clothes, but it took the majority of the series for them to learn something as complicated as Ben's outfit (I mean baggy clothes and shoes - Ben's design itself is wonderfully simple).

That, and I just wanted an excuse to draw Ben in his grandfather's shirt. Because little kids in oversized clothes are cute.

At least in cartoon form.

Middle three panels = fun. Mostly because Ben's annatomy...is annatomy-less. XD

And poor Gwen. She's so fun to pick on.