Talked about this with an MPA that I see often and she mentioned how a lot of MPAs who provide services outside of the MP do so because they can use the MP as a screening area. At the MP they have the power to decide which guys they want to offer their services to and also avoid any guys who might have some red flags. As an SP they don't know much about any new clients which might be a little worrying but still have to provide services for them.
She also mentioned that in the end it is just another way to make money without a middle man and also partly do with the girls meeting guys that they like as well.