>>23346759 (OP)There are multiple factors at play. Popularity of the show, the willingness of the director to go through the process (or how much pull they have), the amount of time and resources the executives are willing to allocate (or which are even available). Perhaps the biggest factor though, is bang for buck.
Tomino, as the creator of Gundam, is considered the literal god of the franchise and thus nobody's going to say no to him when he wants something. There's also cases of the squeaky wheel getting the grease, such as SEED being so wildly popular but also being limited by the technology of the time that a remaster can drum up interest all over again for a big cash cow. AGE didn't actually get a compilation per se, Memories of Eden was just an expansion of 2nd generation to fill in the gaps (again, squeaky wheel getting the grease).
In a lot of cases, it just comes down to re-cutting already-existing animation from aired episodes and cutting out the fat for a "cliff's notes" version, which is the vast majority of compilation movies and which anyone worth their salt will tell you is not worth it (just watch the full series instead). It takes almost zero resources since all the work was already done. G-Reco is a curious case where it was a 2-cour show to being with so the "compilation movies" aren't really a summary so much as a complete re-telling.
Again, it ultimately just comes down to convincing the Bamco executives that it's a good idea, or the executives foisting the idea down upon the director. If they see the opportunity for profit, they'll take it. Of course, with Tomino being the exception that proves the rule.
