Yes, the Manadalay Bay Shark Reef is wonderful but it's only name asset, hardly. Having House of Blues and their beach and Events Center on-site easily gets more media and attendence. And 'Mamma Mia!' certainly hasn't hurt them either.
A not inexpensive theatre ticket on the same property with the clientele of it's THEhotel makes perfect sense. It's simply a far classier location than the home of the shiny lion.
Las Vegas Hotel - Mandalay Bay Hotel and Casino - Press Releases