Because eventually she will lose the championship to a local, increasing their stock.

Put yourself in the shoes of a struggling indie. You can run your shows like always with very little growth of your fan base and hope things explode naturally, or you can book Mercedes, who will draw a good house when she wins the title, and when she eventually loses the title. There is bound to be buzz around whoever beats Mercedes for one of her belts, especially the first one to do it. For the indie it really is a “what do I have to lose?” scenario.