It's clearly a chosen method of writing and/or directing.
This is the Fox superhero movie template: Focus on about three main characters, then make the rest into non-entities who are useful for action scenes and
as token gestures of diversity.
That doesn't work with ensemble/team movies, you have to give people more substance, or trim the cast back a bit.
X2 achieved more balance through character moments (Storm/Kurt convo on jet, the scenes at Bobby's house, Magneto/Pyro, etc, though Deathstrike still got shafted).
They have to make the audience care about all the characters. Or it's just a lot of flashy FX with no substance at all.
I rather suspect they realise Storm got shafted in XM:A and that this is why she's being handed over to New Mutants. But she's been stripped of defining traits, so I'm not sure how they will develop her. They can hardly have her tending plants (and creating micro-storms) in the mansion greenhouse that reminds her of tropical Africa because we never saw her there, only in dry and dusty Egypt. She was just a dark-haired street thief who adored Mystique, and the Mystique worship is not a thing I EVER want to see again.