Raiding is also comparatively a bigger time investment compared to M+ for the (relatively) same rewards, if you don’t count the vault. Having to gather and herd 10 to 25 people at the same time through tactics will just always be more troublesome than gathering 5 people to take through a smoother 20 minute run that doesn’t require too much coordination.
Just to throw my hat in the ring, would it be too much for raid vaults to offer an item of the next level of difficulty if certain conditions were met, similar to how M+ offers much higher level items for comparatively lower difficulty keys?