If rep were account wide, there would be a new wave of qq:
"My guild is forcing me to do all dailies on 20 different characters in order to hit exalted in 3 days, GG on game design blizz"
If Blizzard makes rep gain 1 time only per account:
"Blizzard is removing incentives for us to play alts! GG blizzard for ruining alts"
I don't know, at some point, maybe it's better to simply pay gold to change class? Do it FF14 style.