The reason BG1 had a level 10 cap was because D&D at the time was designed around maxing out at level 10. D&D has evolved since that time to be balanced around maxing out at 20 levels, and therefore BG3 should match that change. On top of that as I said before, if you only get to level 10 out of the 20 possible levels, then mages will be basically useless until level 5-6, which I assume would be halfway through the game.
A level 13 cap would of worked better for me, and some build ideas I have. But I understand people's frustration with only 10 levels in 100 hours of gameplay. For me it's fine, levels will mean something. I'm in it for the adventure. Yeah I like to level, but if it's too much, then it's meaningless. I tried to play Neverwinter Online, and quit after a half an hour. Level 70/ 80 characters really???
And btw, BG1 was based on the second edition rules. In the PHB, which I'm looking at right now, classes capped at 20, not 10.