Fourteen years ago it was time to set my son up for deer hunting,I chose the 7mm-08 Rem.
My decision and reasons for the 7mm-08 Rem for a nine year old
1) Less recoil,I also handload so the first practice batches and first year of hunting ammo were reduced loads which helped a lot for a young shooter.
2) The short action rifles are a bit smaller for younger and smaller hunters
3) I'm also a 7mm(.284) fan when it comes to deer/elk hunting cartridges.
I think this is why it's more popular. Dad's are choosing it for their kids and/or wives as well. After having success for a number of years when young, they are less likely to fall into I need more trap when older. We all carry an affinity for they cartridge we used when younger if we were successful.
I still carry a .243 for deer and lopes. I choose a 708 for my son because elk was in the mix. .280 vs 708 is like having a monster truck to go to the store to pick up milk when a regular non lifted truck will get you there more comfortably.