The last 5-10 years have been primarily reacting to buyers (and choosing not to market sedans more) but there was a specific concerted push to change market opinions toward SUVs and SUV-like vehicles, so I'm inclined to instead blame marketing with a healthy sprinkling of availability.

The last time I bought a sedan I really had to go out of my way to find the only one on the lot that happened to be in my budget and in my desired age/mileage/mechanical shape, and I happened to test drive all 5 sedans that they had on that lot. If I were indifferent towards the body/chasis style I probably never would've looked at the sedans just by pure math of there being easily a hundred or more SUVs, a smattering of trucks and vans and that was about it