Without a doubt, there are many factors contributing to this. Some of the major ones in my mind are price (arena is cheaper than physical play, especially for rotating formats, combined with general increased cost to play making rotating formats much less appealing), and availability (arena is more convenient, especially for older players, and not many standard events in stores anymore).

Standard was also the defaco competitive format for spikes. With no major competitions that are accessible to the general player, even those players moved on.