Tilapia is popular because it is readily accessible, versatile, and cheap.
As of this very moment I can buy a 10lb case for my restaurant from Sysco for 2.74/lb.
Fresh salmon is 9.68/lb, Grouper is 10.43/lb..... today's wholesale prices.
Any nicer restaurant would never have a cheap fish like Tilapia on their menu. And guests should have higher standards than to allow a nicer restaurant (as I would consider the main dining room to be) to skate by serving lower standards of food.
I had Jerk Tilapia from the Buffet on The Joy and it was wonderful - it should not be the fish of the day in the dining room.