Hey Toda.
Just keep in mind, that IWM isn't the only source for Battletech miniatures. There are other retailers out there, Brick and Mortar or otherwise, that often have a half-decent selection of Battletech miniatures. Much of the time, you can find 'mechs that have long since been archived available for standard prices, maybe even pre-pewter-pricejump prices.
Even beyond what's new or not, the other part of the problem is that existing players tend to jump on getting new designs when they come out, but once they have them (one or many), they really don't need to go back, so the trickle of purchases that they get from there on out makes it tough to keep them available.
That, and what keeps the Star League/Succession War designs around so much is the portion of "3025 only, forever" mentality players - who usually refuse to spend any more money beyond what they did 20 years ago, but when they do break down and buy minis, they're mechs available in TRO 3039.