Despite the outward appearances, given the seeming explosive growth of new MLS franchises over the last half-decade, MLS does take its time and due diligence in evaluating prospective investment bids (getting an MLS franchise is not precisely like becoming an owner of a franchise in the other major leagues or most other American sports leagues down the line; you're an "investor-operator" who buys into the league and has the exclusive rights to OPERATE a franchise, but all the league's teams are owned by the umbrella MLS and the league's profits shared amongst them) for likelihood of success upon joining up. Most of the cities that have gotten expansion have had sustained and notable success in lower leagues to help their bid along.
Phoenix Rising FC, the local team, has been making a push for the last several years for consideration. Just hasn't seemed to come to any real fruition yet. While they've been pretty decent in terms of success and attendance, they also haven't been measurably better than cities who've been tapped for MLS (St. Louis, Nashville, Cincy, Charlotte; also counting Sacramento here, even though they dropped their bid earlier this year).
MLS does seem to be on track for getting to 32 (Austin was 27, Charlotte and St. Louis will be 28 and 29 when they join, and Sacramento was due to be 30th), so Phoenix might just get a look-see.
Also, just to be fair and candid: just having a large soccer-mad population doesn't necessarily translate into strong interest for the local product, especially if, say, the population there is Mexican-American and may very well have stronger interest in Liga MX teams even though they're expats.
