The idea I have heard most is that is happening because Google is pushing it for people to use their "cloud" services. You don't need a lot of space for music, use my cloud music streaming service. Don't store your documents in your device, use Google docs. Out of memory in Drive? You should upgrade your plan. And so on.

As a person that likes to carry an awful lot of music allways, it sucks a lot and really limits your choices when buying a new phone.