Many reasons
To be sure, there are many reason. The best reason I've seen so far is due to phones getting bigger.
My experience with tabs is their fragility. Just as easily broken as phones if not more so and just as big a pain the arse to get fixed.
The 3rd reason for sure is price. I can buy a very nice phone or a usable laptop for the price of a tablet.
Various reasons after that are lack of keyboard and portability, Yes portability. How exactly do you carry one around? Not in your pocket, that's for sure. Buy a case? Obviously, only now you are getting into laptop accessory territory so why not just buy a laptop for about the same price but with more power? ...and it comes with a keyboard?
I like tablets. I think they are very cool, but they were never meant to be anything other than a very large niche market.