My theory is that Niantic MASSIVELY overestimated the value of the old Megas for both the Mega Level 4 rollout (bonus is pretty miniscule vs the cost) and the "cater to new/returning players" approach (Megas are useful but only a handful, so a lot of the Mega Raids are still "dead" time).
These events feel like they're struggling more as of late specifically with balancing the "New" and "Old" player appeal. Raids for the big shiny new Meta boss are a good hook for existing spenders, but several of them require resource commitment or a degree of investment in the game to get good mileage out of them (several Adventure Effects for example mostly matter if you play heavily to min-max rewards from catch times). Meanwhile, the wild spawns, lesser stuff like regular Legendaries/Megas are good for new players or more casual ones to get a foothold to play at a higher level and thus "keep up" with their community, but don't have a lot to sell to vets who might already have all of these if they care for them.
Most events have a problem with leaning a bit too hard to one or the other: Go Fests and Tours have frequently been headlining something like the Kyurem or Necrozma fusions, which are strong but require two Mons, a big energy grind, all for a damage increase that you will mostly notice if you play in small groups/solo, without any wild spawns I remember sounding particularly amazing. Wild Area or the Max Out Finale meanwhile has the Mighty Pokemon which make for AMAZING quick builds of decently competent Pokemon, along with GMax Pokemon to enter into the Max Battle Loop, but their raids are usually at most reruns of previous headliners which MAY be appealing, but ultimately don't shake up very much or call for a great deal of grinding (save for Eternatus who is ALL grind but too extreme even for a lot of Vets).
I think the first Wild Area felt like the best balance they have hit with this approach, as the Wild Encounters had a good pool, the Origin Dragons + Primals are a good and bombastic headliner for raids even if rerunning (variety helps so that it's harder for rerunners to get their fill of something they might already have a lot of), and the "new" thing with Toxtricity was relevant very immediately (Electric being a type with middling Max options at the time vs Grimmsnarl being crept-already).
Zygarde raids or Mega Mewtwo definitely would have changed that.
