It's funny how people used the vague XM xmen post from last year to further claim that marvel is blocking all xmen/FF stuff from being made even though that's never actually been the case. And now XM has started releasing new xmen stuff and I don't see those folks around.
Anyway...the character choice is odd but it's possible there's a reason for it. Could be that there's additional costs in using other characters or limits on them. The xmen licensing stuff is crazy complex.
That part is not speculation or hearsay, Disney did block the X Men as much as they could. Comic writers confirmed that they were asked to reduce the X Men (and Fantastic Four) presence to the bone for years, in the comic stories.
So their comic presence was reduced a ton, they also pulled all kind of merchandise, not just high end collectibles (that really are the tip of the iceberg in the merchandise market), but every kind of toy, X Men and F4 were reduced a lot.
Also, it's not just XM. Sideshow had pieces like Emma Frost and Jean Grey approved long before the block and they went on preorder too before the block, but after those, there has been a long hiatus. They had Phoenix approved for years, you can track back a Phoenix comiquette project to 5-6 years ago. You go back 6 months, after the Dark Phoenix PF preorder, and Alex was stating that they had absolutely no other X Men piece planned for 2016. Then a couple months later, they resurrected the Wolverine vs Hulk dio and now she stated that there is a chance for a Green Phoenix PF variant (something that was officially stated to be impossible at the beginning of this year).
About at the same time, XM's X Men pieces were green lighted again too. That's not coincidence, Disney has been slowly releasing the hold throughout the last year.
Problem with the X Men, imho, is that they're not like the Avengers. Most of the main Avengers have been done and re-done, also you can mix and match them up easily, leave out one character, add another, you still have a coherent "Avengers group".
The X Men, yeah you can group them randomly and you still have the "X Men" display, but I think they work better split into their specific teams. Classic teams, Brotherhood team, X Force team, and so on.
And throughout the years, Marvel pretty much made this impossible across various companies by allowing and blocking characters or asking for specific art styles to be followed that made it hard to mix them up for the differences in costumes.
At least for a project like this dio I hoped they'd be a bit more flexible with character choice. The very meaning of a diorama with multiple characters is to display them all together, so character selection is key.