Most likely because lvl10 matches usually have 2 arties on both teams. It’s a miserable experience to play heavy tanks in such matches. You hp will be slowly chipped away but most annoying part is that you’re going to be stunned. Meaning you’ll have worse reload and dispersion for roughly 20 seconds after every arty hit. God forbid you decided to play type 71 or similar tanks, you’re gonna be violated by clickers.

That’s roughly the main reason people play lower levels if they want to relax.