Apparently android phones have overtaken iphones in total volume, but the ipad has some incredibly commanding market share compared to android tablets. Something like 80 or 90%
I think that compared to apple's offerings, android tablets offer more than android phones, and are more customized for the form factor, while the ipad feels like a big ipod touch basically.
I think there are two factors to consider here:
1) Lack of polish. The iPad came to the market with an OS that has been specifically accelerated for touchscreen and graphic usage. The UI might not specifically suit a tablet form factor, or everyone's taste, but the responsiveness is unmatched, and that was absent in most Android tablets sold. In fact, I do believe that's still absent. As I kept preaching... "GPU acceleration", "GPU acceleration", "GPU acceleration",... This isn't a real problem on phones because you obviously don't buy your phone to primarily use it to browse the web, watch movies, view photos, or do anything that's graphically intensive. On a tablet, I think those are the main points, and if the graphical performance is only the least bit lacking, it'll be troublesome. If Android doesn't get a fix by the time high resolution screens appear, the iPad will once again pull ahead and leave the competition even further behind.
2) Lack of support. You buy a phone primarily to make calls, send texts, and occasionally use it to do other things. But with a tablet, you would mostly use it for the applications. This is a big deal, and again, another aspect that Android tablets are lacking behind. It's not that Android has few apps per se, just that iPad has far more apps, and far more games. You can get away with saying that your phone just has to be durable and be able to make calls, but you can't really get away with saying that your tablet only has to be able to browse the web. As users' needs increase with technology, so does their craving for more applications. The problem is... iPad, or the iOS platform as a whole, is a really enticing platform for developers to jump in and start making money. It's a win-win situation where consumers get apps, and developers get money. On Android, it's not as clear what kind of situation it is. Google isn't trying to entice more developers. Android users are less likely to purchase apps and more likely to download freemium titles that include ads. But certain kinds of ads are not acceptable (like ads that pop up in notifications), and other kinds of ads can be blocked easily by users, so the uncertainty kind of pulls possible developers away. Not to say that Android is bad, but that it's a big unknown region. And unknown regions are bad for business projections. In the long run, that may change, but right now, there is little doubt that Android app support, particularly on tablets, is lacking.
So those are the two key points that I think are holding Android tablets back. Many are hoping that Android tablets would catch up, but personally, I don't believe that would happen unless Google addresses those issues soon. If iPad sales are slowing down, it's more likely because around the next few months, a new and better iPad would be announced.
lack of apps? Are you smoking crack? There are plenty of apps for Android Tablets.
Lack of apps compared to the iPad. I'm quite certain there are far more iPad apps than there are in all mobile OSes combined.