This is for several reasons
One: many Android tablets are rubbish. (Will your original Galaxy Tab with a still-quite-good-screen even get even Google's last version of Android, ICS, in a supported build? It hasn't even got the one before that!) This is being solved with the Nexus 7, though, even if it annoys many makers of said rubbish tablets.
Next: lack of USP. This is the maddening one. Google seem to have no plan for tablet Android besides 'iPad, more open, more powerful software environment, cheaper'. And that's not bad but there are lots of great ideas coming up in the Android community (split-screen app model with cut and paste between them), haptic feedback, stylus integration a la Galaxy Note 10.1, gesture palettes on the sides of the tablet right where your fingers are) that are dying because Google won't commit to them. Missed opportunity. Plays right into the above stereotype.
Finally: apps not optimised for tablets. I don't know what Google's plan here is besides wait. That may not be enough given competition from Windows 8.
Google's advantage is that Apple are control freaks. Their problem is that they can't seem to find a way to offer something definitively better, just something that gives you more room to tinker.