I think it's just that fewer are needed, and it competes with 'other types of coding you could be paid to do' as you said - sure domain experience helps a tonne, but basically the 'core competencies' are the same as web/desktop/mobile stuff. Many of us doing the latter trained as EEs even.

So, from that huge pool (big supply) there's plenty for the few that are needed (little demand); enough even that will choose that domain over a better-paying one for interest's sake, so it ends up not having to pay as much.