Better looks -- Fire giants are born to look royal, unlike those skinny albino freaks.
Better role concepts -- One Law, motherfucker!
Better inherent too, and less annoying vulnerability.
Also, fire giants are
classic. Lots of great heroes of the past were fire giants. Frost? Almost none. (That might actually make frost more appealing to play.)
But yeah, don't get why they can't be foresters or mountaineers.
I also don't get why frost giants are skinny. One would want some fat, you know, to survive in the cold.