Don’t want to get into the political side at all, but in different projects I’ve seen that if you fork it to add functionality you need (professionally), and PR to upstream it, the energy to actually get it accepted upstream is sometimes disproportionately high (compared to the implementation effort).
Seeing that in the Nix/FOSS community there’s not even consensus that (or whether?!) flakes are actually a good thing, I could imagine that the resulting resistance to upstream features may be an issue here. So supposing that DetSys would “just want to get things done” first of all, and worry about everything else later, I can understand how things are like they are.
Not sure what to make of the historical thing that Eelco (I think?) wasn’t in favour of making flakes stable anytime soon (before the “rift”), but now DetSys has done just that? But maybe that because of real progress on that topic? In which case upstream might prioritise landing the mentioned PR’s?