It's because Oklahoma City and Minnesota aren't well-known enough in terms of popularity and notoriety compared to, say, teams like Boston, LA, Miami, New York, Golden State, etc, y'know basically teams with commercialized history and notable big names on the roster.
It has nothing to do with the way guys are playing on the court; neither team is a “somebody,” if you get what I'm saying; as franchises, they're just starting to make a name for themselves.
Knicks and Pacers ratings are likely higher for three reasons: New York is one of the biggest market teams with a very well-known popularized history, the Knicks-Pacers matchup has a lot of buzz due to their past history going back 20+ years, and because MSG is a legendary arena.