In SF, at least, the magazine ecosystem associated with short stories has taken a pretty big hit which means new authors tend to not get into the genre that way. Of course, that's a bit self-referential because "Why did that ecosystem largely go away?" and the answer is that I'm not sure. Though I'll note that a fair bit is online these days so maybe new authors felt that was a better way to build their name.

I'd also note that some of the better SF short story writers these days tend to write in a mix of genres and often publish in places like The New Yorker.