The difference is, you actually were able to care about the heroes in Avengers because they were each given their own build up with stand alone movies. Only Wolverine, and to a lesser extent Xavier and Magneto, were ever given that kind of treatment in the X movies. So regardless of how messy and CGI heavy the plots were, at least it involved characters you were familiar with and could root for.

The other thing is that the Thanos plot was built upon since the first Avengers movie and really since Thor 1. Each of the X films falls into the same problem Justice League has in trying to do too much in one or two movies. It is very challenging get the audience engaged with a team of heroes as well as a villain, as well as other side plots, in a single film. By Infinity War we may not have known all about Thanos's motivation but we at least knew who he was and what he wanted, and we could also name all the core Avengers and know their personalities and backstories.

Regardless of what you can say about the writing of the MCU movies (and I think a lot of them are pretty good), they really succeeded in creating a foundation of characters and themes that were continually built and individually focused upon, leading to the climax in IW and Endgame.