It's very hard to get decent media coverage in larger cities for minor league hockey-at least in the US.
The cost of advertising is prohibitive for the budgets of minor league teams as well.
There are exceptions. The AHL Chicago Wolves have been around for a long time in the western suburb of Rosemont. Cincinnati (28th biggest US market) has supported the ECHL Cyclones for the past 9 years. Las Vegas supported the ECHL Wranglers pretty well, but there were issues between the arena and the organization which caused that team to suspend operations after last season. Orlando has an ECHL team.....we'll see where they are in a few years. Milwaukee and Cleveland have had long AHL histories.
In general, minor league hockey has had a very checkered history in large US cities. It's almost impossible to successfully operate a minor league hockey team in a large city if there is already an NHL franchise in the market. In larger markets without an NHL team, minor league and junior teams can survive even without very strong media coverage and high advertising prices, but it's not an ideal situation. Markets like Hershey Pa, Rochester, NY and Fort Wayne Indiana are ideal minor league markets. Large enough to have a corporate base to support a decent sponsorship level and they also have population bases which are sufficient to support a minor league team. Teams in these mid sized markets are generally the best entertainment option as well.