Largely package / feature support. RHEL is clearly betting the farm on OpenShift / OKD. Which is fine, but the decision to depreciate / remove things in RHEL (spice qxl, gluster) are also reflected in CentOS Stream. Even if you want to backport things to Stream as rebuilds of old / existing packages to re-enable some of those features you are now fighting a moving target. Would be easier to target RHEL than Stream if that is the goal.

Fedora has no such depreciation of features, has a larger package library, and more room to grow oVirt into something more compelling. If the decision is made to base on CentOS Stream, might aa well base on Fedora instead as neither is going to have the full enterprise life cycle of RHEL and both will break things here and there. At least with Fedora you don't have to maintain an ever growing list of things to maintain to keep oVirt's feature set in tact.

In short, targeting RHEL over Fedora made sense when CentOS existed as a downstream rebuild, when RHV was a product still, and when the entire oVirt feature set was supported by RHEL. None of those things are true today, and instead of targeting a psuedo RHEL where you still have to maintain a bunch of extra depreciated packages without the lifecycle commitment, Fedora makes more sense to me.

My two cents anyways, for my use case not having Gluster or spice is a breaking change. While i wouldn't mind contributing to oVirt here and there as needed if someone picks up the pieces, i don't have the resources to also maintain the growing list of depreciated / cut features in the base OS.