Because that's who America wants as their next Vice President....another boring, old, arch-conservative white guy that's literally a micro-version of John Boehner.
Honestly, he'll be so concerned on getting re-elected in 2020 and will be such a prime Dem target that he probably won't even realize there is a Presidential election that year. He has to run in a state trending to Democrats thanks to the rise of the black and Hispanic populations of the state, and he'll have to fend off a strong challenge from Cowell, Stein, Foxx or whoever else the NC Dems might get in their bench by then.
