One of the ways that Promotion/Relegation systems work is that regional draw is a lot less of a factor because there aren't big "regional draw teams". The big cities get complex webs of "neighborhood teams" instead; for instance, London alone has dozens of football clubs and the distance between many of them is only a couple of miles. The "regional draw" teams such as national teams are what American sports would refer to as "All-Star" teams that exist briefly, are drafted from local teams, and generally only last for the shape of "a tournament".

The call for Promotion/Relegation in MLB is a bit of asking for an entire rebuild of the current landscape of baseball. What if Denver, Colorado had say five or six smaller teams instead of just the Rockies? They'd all use stadiums smaller than Coors Field. Coors Field would not be filled as often/regularly. There's less money in concessions/merch for any specific team. People in other corners of Colorado are going to feel less like any of those teams represent "the whole state" and are less likely to make trips to Denver to catch games, or even be eyeballs for TV Nielson ratings for Denver team sports. There would be fewer dividends to split to big investors, fewer sponsorships to sell in any one stadium, etc.

All of of which is to say, the big money gaps exist as much because of the hierarchy of the system generates more regular, centralized revenue. Team owners also want a hierarchy of ROI in team investments. The bigger stadiums exist in places with more "regional money", which raises more revenue, which keeps them in a cycle of encouraging bigger stadiums in that area (or moving to an area they think might have more money). It's an unvirtuous cycle of money and breaking that system would require either a lot more money to buy out existing owners or a lot of team owners to agree greed is bad and smaller teams would be a lot chaotic and so more fun for the sport itself.

(ETA: All of which is relevant today in watching the direct struggle play out as we speak between the two big American Men's Soccer organizations. Major League Soccer [MLS] is trying to reproduce the MLB system, including its hierarchy of centralized revenue, and USL is trying trying to build the promotion/relegation system of relatively balanced tiers, with the catch that USL started by trying to be the "down-level" leagues below MLS and so has started on the left foot of following similar stadium and revenue and ownership hierarchies, some of the owners of which don't really want relegation/promotion even if they say they might, and also that USL adding a top tier to directly compete with MLS also has to compete with MLS' first-mover advantage and revenue structure. Women's soccer relationship between NWSL and USL is similar but less chaotic and slightly fewer MLB aspirations from NWSL, given Women's sports in general rarely get to MLB [or NBA or NFL] levels of ROI.)