Because of the $15/hr minimum wage. It will be too high for many small establishments. In many mom-n-pop places, the meal price is low, and a normal tip is one or two dollars. $10 for a home cooked meal, and a $2 tip.
Also, look at buffet places like Golden Corral. Tips there are very small, at best. They have waiters that are tipped employees.
In restaurants that are paying more than $15/hr for wait staff, some will certainly choose to make ends meet by hiring replacement workers at the new base of $15/hr., and getting rid of the higher priced labor. The thinking being that at $15/hr, they will get decent replacement employees, whereas at $8/hr they wouldn't.