First off, I wonder if MvC:I would still be getting the amount of hate it is getting had Dragon Ball FighterZ not been shown off. Yeah, the game still has a ways to go before it gets released later this year, but from early impressions, it has left alot to be desired. That said though, you suddenly have Dragon Ball FighterZ that was shown at the Microsoft Conference and how stylized that looked. So much so that alot of people who reacted to the trailer were jumping up and yelling "MARVEL!" as if Infinite didn't exist. That had to have hurt Capcom a good sum when at E3, people are more hyped at an anime fighter than a long running series that gets people hyped just by saying "When Marvel?" Still, I'm hopeful that we'll still see some nice additions to the game still in terms of the roster (although the leaked roster being real is still going to leave me sore).
From day one Marvel vs Capcom Infinite had to go through an incredible amount of skepticism. Everyone was happy it existed for certain, but the fact that the game seemed to go for a more homogenized art style, the rumors of an MCU focus and the lack of X-Men/Mutants, 2v2 over 3v3 and eventually over 3 months of complete radio silence was worrying. With most people interested having finally played the actual game in some capacity I think people have come to terms with the fact that the game is mechanically sound and exciting in it's own way... but this is well after all the other news - the fact that the game looked hideous, that certain characters looked uglier than expected and lost a huge part of the charm/visual identity from Marvel 3, that the graphical interface is barely even a step above uninspired trite like Playstation All-Stars, that the starting character roster is incredibly uninspired, that it's looking like another Street Fighter V situation in terms of characters and yeah - people were already bitter.
People like Ryce were already proclaiming how awfully disappointing it was looking well before DBFZ got revealed. It just so happens that was the straw that broke the camel's back. Not only was this a game made for huge fans of Dragon Ball (of which there are many) but with there never having been a proper competitively-viable Dragon Ball fighting game people were excited as hell. It has an incredible style that highlights the strengths of it's roots and characters. And a game developed by ArcSys who are perhaps the most technically prolific fighting game devs? Going all the way with a 3v3 set up with assists, DHCs and more? It simultaneously manages to be an original game while being the kind of 3v3 tag team fighter evolution I think most traditional Marvel fans would hope for.
I think both games are going to be great in their own way but I think most people just weren't ready to fully commit to voicing all their grievances about Infinite because for a long time we had to face the fact that this was essentially "better than nothing".