In Ole Anderson's autobiography, he makes several references to the costs of wrestling shows quickly increasing in the 1980s and early 1990s. Although his point is more about the rampant waste in the latter days under Jim Crockett and similar waste once the promotion was sold to Ted Turner, there are two trends that start around this time that seem to drive up the costs for promotions.
1. Wrestler salaries: Ole makes a few references to this, but never tackles the bigger question directly. Before Vince McMahon started raiding territories for talent, a point Ole says left him largely with non-main event caliber talent, written contracts unnecessary. The whole point of the NWA was to discourage competition after all. Now, suddenly written contracts or at the very least guaranteed minimums per show become the norm. If shows don't sell well, the debts will pile up rather quickly. While the WWF was in a better position to offer larger contracts, other promotions, especially World Championship Wrestling (WCW) by 1988 on, has to follow suit or watch their talent leave (many, including Arn Anderson do leave briefly). Want to keep Ric Flair from finally jumping ship? Throw money at him! While Flair was arguably worth the additional costs, this creates a trickle down effect, especially when those making the decisions on wrestler salaries are ignorant of the costs associated with wrestling.
2. Television production: As mentioned before, the move away from cheap studio productions to slicker, multi-camera productions cost more money. While one may want to cut corners here, fans now with access to other promotions via cable are likely going to flock to the promotion that looks more impressive. This continues today with the pyrotechnics, big screens, and other non-essentials. To discard these additions risks turning off fans and being labeled bush league. To a certain extent, this ratcheting up the costs may have been intentional by Vince McMahon during the WWF expansion. Other than Ted Turner, who seemed reluctant to spend exorbitantly until the late 90s, other promoters simply could not match the costs.
The take-home here is that by the end of the 1980s/early 1990s, the remaining territories simply cannot compete the high start-up costs to compete with the WWF dissuade many would-be entrants to the market.