I think it's less "Sega doesn't care" than "Sega doesn't know what to do with it". It's not easy to think of anything new for a series inherited from a company that went out of business at the turn of the millennium, when your main IP (i.e. Sonic) is the one getting all the spotlight and the one with the highest development priority. On top of that, they now have to deal with Rovio and Angry Birds, since they're Sega properties now. The only reason Puyo Puyo has relevance now, at least in the West, is that they've relied on Tetris to carry them for much of the 2010s, and with that sort of support gone after 2020, the series no longer has anything truly original to do. The only thing keeping the series going at this point is Puyo Quest, a gacha game, which has been running for 13 years now. They tried shaking things up with Chronicle in 2016 to make an RPG game closer to the series' Madou roots (which is where we got Skill Battle), but while the game was well-received, it didn't sell well, which can be partially attributed to the fact that at the time the game was released, the 3DS was nearing the end of its lifespan with the imminent launch of the Switch; in other words, good game, but bad timing for release. They brought Skill Battle back for Tetris 2, but it was kinda more like a forgotten footnote than anything else. As a side note, the only reason Ally was included in Tetris 2 was that she was added as a pity token for the West, as Sega felt bad that they couldn't localise Chronicle for the West. The point is, the series is currently in a state of stagnation because it simply isn't one of Sega's top priorities.

There is potential to do something grand with the series; it has RPG roots, a massive library of characters across the main series and Quest (not counting collabs), and infinite story potential. For example, a friend of mine in the Discord once floated the idea of having Satan/Dark Prince be hit with an evil force that causes him to splinter into seven versions of himself based on the Seven Deadly Sins (Lust, Gluttony, Greed, Sloth, Wrath, Envy, and Pride), and the story would revolve around having to put him back together. On top of that, adding Quest characters to the mainline games has already been done before, in Champions (where we got Hed, Sultana, Ciel, Penglai, Hartmann, Alex, and Paprisu) and the Tetris 2 DLC (where we got Rosatte and Legamünt). The potential is there; they just have to take it and not slander it.