Hate to tell you but that's not the case.
It's sales. Lego can make any non xmen movie set they want. Yes, marvel has to approve it. If marvel was blocking "across the board" there would be nothing. That's not the case. There are xmen in the marvel universe 3.75 inch line. There are xmen in the 12 inch line. There are xmen costumes for Halloween, and xmen ornaments. There's are ithe xmen items. The only thing marvel is limiting is movie stuff, for xmen, because of license issues between fox, the actors, and marvel.
Yes GotG is unproven, as a toy franchise. However xmen isn't. And it's proven to be a slow seller. The two xmen sets they have done are two of the worst hero sets lego has produced.
The GotG film has done better then all the xmen flicks. In total tickets used (not how much a ticket costs, or adjusted price or box office take ect but total movie tickets used) the highest ranked xmen flick is 110 the lowest 550. In comparison avengers is 29 and iron man is 44. GotG is in the low 50s. Here are a few other flicks that did more tickets used...101 Dalmations, I am Legend, Bond Goldfinger, home alone, batman forever(every batman movie in fact). So the perception that xmen is such a big franchise held down by marvel is not supported by the facts. Go back to before dOFP, as it's the first movie (actually the strain started during the pre planning for DOFP and Avengers 2 because of a request between marvel and fox that was denied by Fox and pissed marvel off..which led to fox not getting licensing rights, which led to marvel not doing any additional merch for the movie. And the movie only) anyway..back before DOFP or even further back to before marvel had its own studio in 2006ish there was not near as much xmen merch as there was batman or Spider-Man.
It's sales. Always has been.
If marvel was limiting to screw with fox then they would just stop all licensed merch. Not let some. That would do no good at all. And makes zero sense. It's like saying I'm going on strike...but still work four days a week.
Everyone seems to think xmen is this huge franchise. They do well in comics, yes. But they have never done gang busters in movie or merch. Compared to Superman or batman or iron man or avengers.
I've said this before but maybe u didn't see...
In total ticket sales (number of tickets used not how much they cost or any of that)the xmen films don't have a single flick in the top 100 all time. Home alone has more. The dark crystal.