The main reason is poor app quality and selection as most Android tablet apps are upsized phone apps.
Also poor build quality, reliability, and support.
Inferior screens.
No LTE except for one that I can think of.
Terrible interface that is different for the sake of being different.
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If only there were a zillion keyboard solutions for the iPad.
When the iPad 2 was released the Android competition had better screens and LTE. LTE and a decent screen is only a recent addition to the device.
I have a Xoom and just sold my iPad 2 for the new iPad (wifi).
So I have a device thats less the 6 months old and an android tablet that's over a year and a half old. And the only thing that's better on the iPad is it's screen. The cameras aren't as good, there is no camera flash, there is no real USB hosting, mono speaker, no sd card. Build quality and reliability is the same, however I know there are some cheap feeling android tablet.
I do like my iPad but even in its 3rd iteration it lacks in some things.