Why aren't tablets supersets of smartphones?
Why would you want to carry both if you can just carry a tablet?
1. You can't put your tablet in your pocket; it needs to go into a backpack or case or purse of some sort.
2. Nobody wants to hold a tablet up to their ear to make a phone call.
Edit: (Almost) nobody wants to hold a tablet up to their face to take a picture.
Because the tablet is replacing a laptop. Instead of carrying a phone and a laptop, you're carrying a phone and a tablet.
Because a tablet is a subset to smartphones? Just carry a smartphone and a laptop.
They don't make pockets big enough.