10-14 used to be more common 20 or so years ago. Plenty of Horn Book reviews listed that age range. Lately, people on Twitter are trying to drum up renewed support for it, saying readers age 12-15 are being lost, which I believe.
Nothing's going to change unless publishers decide they can sell it. Until they do, it doesn't make any sense for agents to take it on. What's made available to the public is controlled by the houses, not agents or writers. I'm hoping publishing might listen to librarians on this. If editors start asking for it, then agents can start, and that'll open things up.
I figure getting published is so hard, and I've written more of my share of stuff that bucks the market ("too quiet", historical, too much faith element, etc.) that I'm no longer willing to build in for myself another obstacle. I make my MCs 12 years old. Making them 10 would make sense if you were aiming for the lower end of MG.