I think this is the most important point. Infection is a game type, which can be worked up in Forge and dropped into any map. Griffball requires a flat space, hammers and a bomb.
Invasion requires enormous maps built just for it, unique among all of Halo's game types, ever. And Reach clearly didn't have the resources to spare. It's apparent that Invasion's gain was BTB's loss, and we're still not recovered from that decision. For the benefit of one game type, we lost out on good CTF, Assault, 1-Flag, 1-Bomb and Slayer in a BTB setting.
I'm not against Invasion as a concept returning, but were it to do so, I hope it is in one of two ways. The first is properly resourced, with a development thought process that first asks whether including it would damage any of the other core game types or maps needed to support them. If the answer is yes, pitch it.
The other is to reimagine it as a free form game type that lets you string others together in phases, which is really what Invasion's contribution to things is. Add a game type that lets us have Phase 1, Phase 2, etc. with the ability to string game types together (Phase 2: CTF) and you basically have Invasion (which is just 1-bomb, 1-flag, and Territories strung together, after all). Build that out as a game type option. It would be neat, if the resources existed for it. But it's nothing that needs entire maps built just for it.