MLS is uniquely situated in that it spans two countries.
My theory is that the MLS will continue to quietly expand until they have 33 clubs, at which point the Canadian teams will split off and the USA teams will be divided up into two 15-club divisions that practice promotion and relegation. I suspect that the CPL was created to be the eventual landing spot for the Canadian MLS clubs, which would explain why they get such favorable Champions League qualification opportunities not enjoyed by the USL.
I don't have any evidence of this wild conspiracy theory that I've concocted, but the shoe certainly fits.