Cost of development is much lower on iOS, so for a small company they are far more likely to be able to afford to develop for the iPad. Not to mention the decreased cost if they first complete a MacOS X port.
Android is fragmented, with different hardware with different supported feature sets (think early Windows Direct X days with so many video cards with different hardware accelerated features that a game developer had to account for each individual video card.)
A lot of well known app developers have came out against Android, but I'm sure google will eventually solve the problem. If nothing else by required all the phone manufacturers to sell the same hardware.
Angry Birds developer
PDF support is much better on iPad compared to Android combined with the ease of syncing new files to the iPad, I think the iPad will continue to maintain the top platform for gaming (tabletop) for a long time.