The trend for growing truck is mostly due to EPA fleet fuel efficiency regulations. If you make a small truck, it gets classified as a car, so it counts for car fleet efficiency. That’s also why you see decline in sedans and rise in relatively small crossovers SUVs: these also count as light trucks, not cars, for fuel efficiency purposes.
It's also the "footprint" model of fuel economy for CAFE. The required MPG for a truck is based on the area of the rectangle made by the wheels. This means that for 2021, a larger F-150 is expected to get 25mpg, but something like the old Chevy S-10 would be expected to get 41.8mpg. [1]
The automakers have determined that this is not practically achievable, and so you can't buy a truck like the old S-10 or a pre-Tacoma Toyota pickup anymore.
It's more that people prioritize incentives for objective "A" and are totally surprised when it comes at some cost to "lesser" objective "B." In this case, being "environmentally friendly" vs being "safe."
I think a lot of "environmentally conscious" people don't realize that more and more of these types of environmental regulations will come at some direct cost in terms human-safety.
And also that a surprising number of politicians/activists/people are so committed to the dogma that they don't have any problem with that.
Higher, larger pickup trucks did not come about as a result of safety requirements, they came about from fuel efficiency requirements (or needing to get around them).
So net result of indirect regulations to decrease fuel consumption is increased fuel consumption, and more dangerous vehicles for everyone to contend with who is not inside as large of a vehicle.
Simply making people pay more for fuel would have incentivized them to prioritize what kind of vehicle to buy, and make it possible for smaller pickups to still exist and/or a rental market for pickups to flourish.
