Convenience and price both play a role. Like it or not, streaming music has most content on each of the major services, is fairly cheap, and is very convenient. It's not a shift I'm entirely happy with and I keep a foot in each world (owning and streaming) at least in part because I already have a big music collection.

Video and other content is much more of a mixed bag with very fractured availability and simple lack of availability under a subscription for a lot of content.