I think this is a perceptual thing that isn't necessarily supported by the data. A quick google got me to this page:
https://howmuch.net/articles/sports-leagues-by-revenue
And this graphic:
These numbers are from 2015, but I don't believe there has actually been a significant decline in revenues since then. Attendance and ratings may be different, but with the advent of streaming services and less and less 'must-watch' television, networks are scrambling for advertisers and sports is one of the last bastions of 'you need to watch while it's happening.'
Also, the NHL - globally - is incredibly strong, beating out the top leagues in Germany, Spain, Italy, France, and Japan. In fact, if you combine the revenues of the Bundesliga and Nippon baseball, it comes out to exactly what the NHL made in 2015. Germany and Japan combined have a population base of 209 million people, and neither of those leagues have to face the kind of competition the NHL does in the NFL, NBA, and MLB.