It was always there. It was probably just a contractual thing.
I think it was a combination of bandwidth and contractual thing. I have no idea on Fox Sports, NESN, or some of the other RSN's, but back when the Comcast Sportsnet (now NBC Sports Net) were renegotiated back in Dec. 2014, the RSN's were HD only during the games. At the end of 2015/beginning of 2016 is when the continental transponders on WA went from QPSK to 8PSK and channels were moved all around. I want to say it was in March of 2016 that the RSN's went to being on 16 hours a day, then off air 8 hours. At that point they had the bandwidth and my guess is that is when the contractual part came into play or else they would have gone HD 24/7 then. The other thing that happened was Dish went from 6-8 HD channels per transponder to now they have 9-10 HD channels per transponder - so they crammed more channels in the same amount of space.
Just thinking back to when I first got Dish back in Dec. 2009 - I remember how there was only bandwidth available for 8 HD games at a time and how often the Bulls weren't in HD.