Short fiction was being bought by magazines with large reader bases. Magazines have essentially died as a medium over the last twenty years, and fiction magazines were on their way out well before then.

If you've got a larger reader base and lots of competition, you can pay a lot for content. If you don't, you can't. The various TV subscription services are playing the same game that the sci-fi magazines used to; they pay a huge amount to produce content for recurring revenue, in fairly tight competition with the other streaming services to have the best stuff. (Think the expanse vs the mandalorian vs unbounded quantities of star trek.) The primary medium for consuming sci-fi changed as it went more mainstream, but also magazines died generally.