What the MCU did right, was have a plan from day one for this big ass cinematic Universe.
With most studios, thinking sequels before even launching your first film can often lead to a poor start and a series never taking off, but however they thought it out, the MCU succeeded in launching and maintaining it.
Fox has poorly handled the X-Men series for a long time, and I'm not getting into the quality of individual movies, but the disjointed overall world.
After Last Stand more or less concluded a trilogy, they got greedy and wanted to make more films off the success, there was a plan for a series of origin films, first Wolverine, then I believe Magneto was to come, but we saw how Wolverine was received, so solo origin films evolved to First Class, but First Class doesn't 100% follow the timeline and almost comes off as a reboot, then the Wolverine comes and is almost a sequel to Last Stand so it's like, we're rebooted and now going back, then you get Days of Future Past which toys more with timelines and Apocalypse.
They're trying to build an XCU at Fox but the never planned to and their make it up on the fly approach isn't doing as well.