My fam worked for marvel, and since some of the managers knew me since i was a baby, i was befriending some of the advert managers and others, always asking them about stuff like this, lemme give ya the details;
So years ago, with infinite, Marvel thought even without the effort or style or X-Men of umvc3, the banger would be a banger because it was just, mvc. We all know how that happened. What this basically is, is another testing of the waters. The main reason it couldn't be considered before was because FOX owned the licensee for the X-Men and other properties like venom. Rights were all over the place. Only recently a few years ago, did they start doing signings for Marvel to have some permissions and rights (This is especially evident with characters like Venom. It's why they even had Eddy Brock technically in the MCU for a brief duration, to leave the symbiote for Tom Holland Peter Parker. The next movie will most likely be Ultimate spider man with the black suit) Due to this, since there was no reason not to, to test interest with Mahvel, they are releasing this collection to test interest, sales, popularity, etc.
Very good chance we possibly get a shot at MVC4 if this is massively sold.