It wasn’t when Blu-Ray came around. The decision was made in 2005 in order to commemorate TOS’s 40th anniversary and re-entry into the US syndication market. And the original broadcasts of TOS-R were in 480i/576i SD sourced from the new 1080p masters. Also, originally, CBS withdrew Enterprise from syndication for TOS-R, so CBS was able to charge the syndication rates in both SD and HD for TOS-R that they had been charging for Enterprise (as the used the same contract that they had with Enterprise).Paramount only has themselves to blame as they've fleeced the fanbase so much over the years with these home releases. When TOS first came to DVD they didn't even start with seasons. They sold two episode DVD discs that weren't even given a proper remaster. It took them awhile to release all the episodes, but once that was done they moved to the season sets with TNG, DS9 and Voyager before going back to TOS to release season sets. Then when Blu ray came around they decided to do a proper remaster of TOS and released the season sets again. That's not counting the carious collectors or fan collective sets that have been released over the years. If you go to Amazon and type in Star Trek DVD you'll see numerous releases for the various series and you have to read carefully because you never know if that TOS set is the remastered one or not. At some point, the fanbase decided enough was enough.
The first Blu-Ray’s weren’t released until 2009. Season 1 had been released on the HD-DVD/480p DVD format in 2008, and 2&3 on 480p DVD in 2008 and 2009. Also, since 2006, CBS has phased out the original 1960’s prints of TOS, with the exception of the Blu-Rays (and iTunes), so the only available version is the Remastered edition. TNG-R on the other hand, went straight to Blu-Ray—-a format that still has not caught on as well as DVD. And CBS never released TNG-R on DVD, so all DVD releases have been the original 480i masters from the 80’s and 90’s. Also, we do not know how well the Blu-Rays of TNG-R sold after Season 4 as CBS never released that information. All that is from guesswork and third-party contractors who are not involved in the internal discussions at CBS and only know what they are told, which could be anything from partial-truths to outright false statements. And right now, CBS is making far more money on TNG-R from syndication fees than they ever could from DVD, Blu-Ray and streaming (really on Paramount plus, CBS is paying themselves).
However, it is curious, from a business perspective, as to why they have not Remastered DS9 and Voyager, as in Syndication CBS is essentially giving those series away for free, because the market for old SD-only shows is niche (and DS9 and Voyager make up almost 50% of all Trek produced to date—-352 hours out of 870 TV hours not counting theatrics movies/half of 870 is 435). Also, CBS keeps giving streaming services files that are 640x240, since the files are recompressed versions of the DVD files (that, were compressed versions of the D2 NTSC Composite videotapes—-Star Trek was rendered in NTSC Composite from 1987-2001 with the switch from analog composite to digital composite occurring in 1990) that have had one of their fields removed, and the remaining field doubled, whereas TV stations that air either series are up scaling them from composite 480i SD tape, so they are using all the available information, and they are processing the colour on modern equipment, versus 2002-2003 equipment for the DVD’s/streaming files. Really CBS would do better to re-edit DS9 & Voyager into HD, than just keeping them in 480i composite video. Right now it’s like if Disney only allowed 16mm Black-and-white prints of their 1950’s “Davy Crockett” series to be used for syndication and home video/streaming, while letting the 35mm colour prints to not be used. Disney would not get the prices they wanted. Same with CBS, they are losing money on DS9 and Voyager right now because they are locked into SD. Sure both shows left the air in the black, however as time passes by, stations are going to start not wanting to air SD-only shows because the machines are old and are wearing out and parts are getting harder to find. With TOS-R and TNG-R, they are now future-proofed. DS9 and Voyager are in antiquated condition.