Most people likely have one brand of theater around them. For them, there isn't much difference in availability between MoviePass and that chain's offering.
The population is heavily weighted to urban areas, and urban areas tend to have multiple major chains, multiple local independents, and possibly regional chains.
I doubt very much that most people only have a single “brand of theater” (or even single firm, since some firms of multiple brands) around them, for ant reasonable definition of “around”.
Where I'm at, while there are a couple AMC theaters, the place is heavily dominated by Edwards/Regal theaters. While yes, there would be more places that accept MoviePass, effectively for me, there wouldn't be much difference between availability. In fact, if the Edwards/Regal one were to offer something similar to AMC (allowing IMAX and other premium showings), it would be more appealing to me.
I doubt very much that most people only have a single “brand of theater” (or even single firm, since some firms of multiple brands) around them, for ant reasonable definition of “around”.