Maybe it’s because most Diablo fans didn’t actually play Diablo Immortal.
But D:I actually has way more to do than D4, and it’s not really close.
The PvP is better. The clan support is better (actual clan battles! Become an Immortal!). The achievements are better (there’s a bestiary!), the dungeons are better (combat on moving rafts, rotating bonus rewards, target farm set pieces, no backtracking).
Not to mention the multiplayer raid bosses.
And honestly i haven’t even mentioned everything because I forget the names of all the features but there IS more than I said here.
Hopefully one day D4 gets what D:I had on release.
I poked at D:I when it came out. Within about a month of release. It had hardly anything to do. There was the story, and then a smattering of “end game” content that was clearly aimed at just pushing microtransactions; mainly PvP, because it was (and is) the only thing to do in the game. It’s a Pay-to-win game with a competitive core loop.
D:I currently has more than D4 does, because it’s been out longer, and it’s an entirely different game, that’s based on predatory practices via MTX. D4 has only been out a month, and just like D:I, D3, and D2, it’s kinda barren on launch, because Blizz doesn’t design these games around the people who blitz to end game.
Is that a fault of Blizz? Arguably, yes. They could have more on release to do at end game, but choose not to in favor of seasons releasing next month, moving us straight into a seasonal loop, instead of waiting for months like they did with previous games.