It’s not the same to travel standing up (i.e. not having an assigned seat) than to be able to stand up (i.e. have an assigned seat).
I’d say that having to buy a ticket with an assigned seat in high-speed trains in Spain has been always been that way.
However, short-route trains (Cercanías) don’t have assigned seat, but they are not high-speed trains.
It will be to keep the quality of the service high, reduce boarding times, or something like that.