Conventional hoppers leak too much for grain. Sealing the hopper doors was a bigger pain than using grain doors.
I think the last boxcar-hauled grain in Washington was the ex-GN Mansfield Branch, near Wenatchee. It was laid, in part, with used rail from the original GN mainline, and nothing bigger than GP-7s and 50-ton boxcars could run on it. The elevators switched to trucks, and the line was abandoned. It was apparently still in use in 1983, and there were LOTS of old boxcars, most in pre-BN paint, stored east of Wenatchee, waiting for the grain rush, when Mom and I were out there for her HS reunion.
Many of the country elevators I remember could load either boxcars or hoppers. All they did was add another chute for the hopper, and a new gate position to control which one the grain went down. That's also how they routed grain from the truck dump to the various bins.