Because there's now a strong demand for one? You only have to look at the iPod range to see that they don't like to leave a niche unanswered.
This. iPod Touch still has a niche though.
I was considering this morning what Android would have to do in the tablet space so that
I would consider it to have equaled the iPad. I came up with this:
1) I'd need to be able to fill the homescreen with apps that I actually use and that span the things I want to do. That includes games (big lack), readers (plenty of those on Android actually so that's covered), videos (ditto), kids educational apps (huge lack here), cooking, and probably some other stuff I can't think of. Currently the wife's KF is used for internetting and not much else, because I just can't find the content for her wrt really great apps.
2) Improved kinaesthetics, or perhaps I mean physics, or whatever you call the feel of scrolling and flicking and swiping. In iOS it feels tangible. You intuitively know if you flick up how far it will scroll. Android still feels like it doesn't have a very solid physics engine underlying it. Multitouch gestures on iPad are also brilliant and feel precisely right - grabbing in to close an app, pulling up to see the multitask bar, left or right to change apps on the fly.
3) A keyboard on which I can touch type on the glass. Believe it or not, I frequently use my iPad for notetaking. The keyboard really is good enough for me to hammer on. I won't be writing novels but for meetings, training videos etc, it works. There's no button lag, the multitouch is accurate enough to follow me, and the autocorrect fills in enough of my errors to be usable. It lends an engineering weight to the whole system. Comparatively, a more attractive UI is flimflammery to me.
4) Identical apps to perform and feel identically, or at least, just as good as one another. This is a biggie. I use Isilo reader on both; and on iPad or even since iPhone days it has been a smooth scrolling experience with clear fonts. On Android it jerks and huffs and puffs and is aliased and looks like Times New Roman. Android tablet apps seem to get the short thrift of developer attention. They arrive later and often halfbaked.