If we look at the time frame NBC had the Stanley Cup playoffs, they lucked out from 2008-15. In 2006 and 2007, NBC recorded some of their worst ratings ever. I believe the 2013 Stanley Cup Finals (Chicago vs Boston), had the most viewers per game.
With the exception of 2019, the finals matches from 2016-22 were not that impressive. I would not judge the TBS/ESPN contract by the first two years of the contract. The big market teams will start to make appearances, and things will even out eventually.
The 2008 - 2019 streak is unreal:
2008/2009 -Penguins v Red Wings (six games and seven game rematch)
2010 - Blackhawks v Flyers (six games)
2011 - Bruins v Canucks (seven games)
2012 - Devils v Kings (probably the worst matchup for the ratings of this era still six games)
2013 - Blackhawks v Bruins (probably the dream matchup)
2014 - Rangers v Kings (five games so the most disappointing series of this era for NBC)
2015 - Blackhawks v Tampa Bay (six games)
2016/2017 - Penguins v Sharks/Penguins v Predators (both six games)
2018 - Capitals v Golden Knights (Vegas first year was good for the ratings)
2019 - Bruins v Blues (seven games)
NBC was really lucky that time.