Their shots and traps are powered by magic, they shoot magical projectiles or explosions which is powered by magic. ( arcane shot, explosive shot, cobra shot, black arrow, all magic in lore) eve their traps are powered by magic. they do it on their own. Concussive Shot, serpent sting, scatter shot, are powered by magic. meaning it takes mana to power those things. hunters are a magic based class because they use magic to power their shots on their own. same with mend and reviving pets also magic. bandaging up your pets while they are getting hack and slashed makes no sense it can only be magic. So it makes sense that they use mana over focus. using focus doesn't make it feel like a hunter but an ADHD fanatic that's having a hard time managing his focus over his enemies,
Hunter: GUYS GUYS I GOT THIS ONE gonna kill that monk!!!
Hunter: CRAP IM ALMOST OUT OF FOCUS ALREADY!!!11
(runs out of focus, chases butterfly)
Hunter: uh, so what were we doing again?
(gets killed)
And of course mana Is a lot better than focus in game, but that's saved for another discussion.