Two reasons, I'd say.
First, it was their concept for the line from the beginning, when their production and customer base was more limited. The idea was to provide larger production runs of designs that were on par with Vox & Anso customs, but that also still had some measure of exclusivity. Vox in particular has always done that with his custom models. He makes a handful (sometimes literally just 5 or 10) and then that's it. They've stuck to their original model which seeks to balance availability and exclusivity and I like the consistency that they've displayed by doing so.
Second, the exclusive runs cater to their enthusiast/core fan base and also provide them data one which designs have extremely high demand, and this fuels what they can then decide to adapt into continuous production. Not all of the GM line was super popular. The GM2, GM4, & GM7 for example. Some of those sat on the site for quite a while, and I doubt we'll be seeing Ace versions of any of them. Other designs that were clearly carrying a lot of mass appeal got worked into the ACE line, some of which are superior to the original GM version. The biggest example of this the Ace Sonoma which is definitely and evolution over the GM3. I'm definitely much happier having the Sonoma than simply having GM3s indefinitely available. A design based on the GM6 is also in the works, and given how popular the GM8 was, I'm sure we'll see that eventually as well.
I realize that FOMO is a thing and that some people just don't like limited runs of anything, and I can sympathize with that. But the Giant Mouse model definitely seems to work really well for them and it makes a lot of sense to me.
