Sometimes it is the attitude that leads to being dropped - I've seen that where somebody is undoubtedly talented, but they only do what they want to do, don't support other players, get involved in excluding or making nasty comments to others, aren't polite to helpers, that kind of thing.
And not all coaches fancy telling upset Mums 'Yes, your kid is great at x sport, but they're an irritating, lazy git who frequently tells others that they're never going to be as good as he is or points out x has cheaper shoes on, so I'd rather risk losing than reward him with utter confidence that he'll always be picked.
Other times, it's to try out new combinations - such as 'next week's team have a lot of much taller attackers. I think we'll try using A who is tall or B who is small, fast and good on the ground'.
Or even 'X gives the impression of taking rejection hard and gives up when something isn't easy first time. They're good, but they need to learn to deal with it. I'll do it this time to give them the chance to step up/handle it before Y game'.
Not pleasant, whatever the reason. But he'd probably benefit from asking what he needs to work and improve on - and do it.