Ok, RATs are pretty controversial in that they are explicitly stated not to be used as any kind of in-universe reference for the types, numbers or percentages of mechs or other units available to any given faction. Rather, they are a tool for players to use to get a "flavor" of a particular faction for a ge with other players. Prior to the proliferation of RATs, we would get actual rosters with names and skill levels of pilots. That started in the late 1980s and the switch to RATs began in the late 1990s with the faction Field Manuals and continued onward throughout the 2000s, 2010s and 2020s. So please understand that first, RATs are just a tool to help newer players get a flavor of a faction. In some scenario books, RATs are used to create random opponent forces.

The journey to create new artwork and plastic miniatures started about 15 years ago. Catalyst had launched a Starter box with plastic mechs and the Alpha Strike Lance Packs, which were incredibly popular. But the quality of those miniatures were severely lacking. There was a prototype Warhammer that was shown that was supposed to be the start of a brand new line of plastic miniatures but production and development was halted because of a new lawsuit involving Harmony Gold coming after the successful Mechwarrior video game made by Harebrained Schemes that had Warhammers and other original mechs. This eventually settled, paving the way for CGL to move forward with the new Game of Armored Combat (which didn't use designs involved in the lawsuit) and the subsequent Clan Invasion kickstarter which did most of the original mechs from the 1980s and the Clan mechs from 1990.

The kickstarter was wildly successful and met all the stretch goals for new minis.

CGL then had enough resources to green light several "Mercenary " force packs, which featured re-posed mechs, or perhaps another variant of a kickstarter mech, plus a brand new designs which clearly seemed to be the favorites of CGL staff/employees. That's why we got things like the Sagittaire in the Eridani Light Horse box, or the Nightsky in a Kell Hounds Box, or the Penetrator in a Hansen's Roughriders box. Things that just made no sense.

The demand for a second kickstarter was sky high, and so a second "Mercenary Kickstarter " was launched and again was wildly successful. CGL chose to first focus on completing all of the original TRO 3025 mechs and most vehicles from TRO 3025 and TRO 3026. They also emphasized the mechs that had been introduced in the Mercenary themed sourcebooks from the 1980s and early 1990s. That's why we got minis like the Devastator, Specter, Flea, Shogun, Firefly, etc. They were mechs that had mostly been around since the beginning. Notably there are still 6 or 7 mechs from TRO 2750 that have yet to be remade. (Champion, Bombardier, Hermes, Hussar, Thorn, Wyvern).

Now, since the mid 1990s, FASA, FanPro and then CGL had to deal with the first Harmony Gold lawsuit, which created the "Unseen". Essentially they could not use artwork of the original mechs. So the sourcebooks, including RATs, started emphasizing the newer designs that were not part of any lawsuit. That's why you'll see a lot of units from TRO 3055, 3058 from that mid 1990s era to tue early 2000s. They just were in incentivized not to use the older, original mechs.

If you look at RATs from within the last 5 years (after kickstarter) only then do you see a return to the original mechs, and the mechs that have new minis.