Because it's hard to quality control. And the responsibility for that is unclear.
Ah, so if they had a competent company running it, it might have been successful.
It is possible to do proper quality control on a nationwide basis: I can walk into any McDonalds in the UK and the food will be the same and most of them aren't even run by McDonalds directly; they are franchisees.
Same applies to Burger King and Five Guys and many other successful chains.