There are plenty of minis with small bases...I assume you mean tiny and smaller?
I don't have an official answer, but it could be the difficulty of painting something that small. Maybe they wanted to get more experience before tackling them.
It could also be they've decided it isn't worth it. In my experience D&D minis with Tiny bases simply didn't work all that well.
I have to agree with bugleyman. All my Pathfinder Goblins (I am looking at some right now) are on small bases. A lot of others are as well.
As for Tiny, often the minis have a hard time standing up on their own as they become top heavy on their own. The few DDMs on Tiny bases had that issue. The DDM Bat Familiar or Stirge come to mind as examples.
This fella is tiny on a smal base:
http://paizo.com/products/btpy91ku?Pathfinder-Battles-Skull-Shackles-Bloodb ug
with a smaller base it would fall over and probbably still not fit in the same 1/2 inch square as a medium or smal base figure
I was going to say there are ones on small bases...
When previewing Gimcrak from the Wrath of the Righteous line, Erik Mona stated: "Gimcrak is a Small (really Tiny, but on a Small base for stability) creature slated at the rare rarity." So the base has been purposely chosen for stability.
Yeah, so I went back and checked and there are clearly plenty of smalls. I think my brain was crossing minis and pawns when I posted. There are no small pawn bases, right?
Correct! In fact, at first I thought you were talking about the pawns for that exact reason. :)