Why? Well, after a year of Maemo development, the built-in email program on the 800 is useless with IMAP.
The version of Opera is so quirky that the forums generally recommend Minimo -- but Minimo is so unfinished that a single button press can blow up the whole app.
Installing software is much easier with repositories, but the forums are still full of users finding "x component not found" error messages.
And then there's the whole "it's not a PDA, so we're not going to include decent contact or calendar software" thread, or the challenges of the handwriting recognition, or...
Don't get me wrong -- I love the screen on the 800, and the speed, and I love playing around with it, but for every day use, the E70 works, while, more often than not, the 800 doesn't.
I think Walter Mossberg's review from last year still holds true -- the 800 is a marvelous piece of hardware that is held back by unfinished software.