The problem with food businesses is Shabboses and Peseach. I think most kosher Donuts are not owned by jews. so these are not problems for them.
In general the most successful franchises are food businesses and they are not usually kosher so that is not really an option. There was Kosher Subway for a while, but they all failed.
There was also a Kosher Nathans Hot dogs for a while in brooklyn and that failed too