TL;DR:
Standard has gotten less fun as players [via Arena] create solved formats at a breakneck pace unseen before in Magic history. Combine that with a shuttering of play due to a pandemic that led to players finding non-rotating formats they could invest in due to uncertainty for when Magic play would return and you get all-time lows in Standard play.
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.
