In Arena (and pvp in general) its burst damage that counts, along with being able to survive burst damage.
PvP-gear is also aimed for that (lots of crit, stamina and resillience)
MM has Silencing shot wich is pretty awesome for PvP because it can stop heals etc.
MM also has a lot of AP bonuses through talents so they hit hard. (barrage and imp. barrage also help with big shots)
And 3rd, MM has acces to imp. wingclip, survivalist and Surefooted along with a reduced trap resistance with SV as secondary tree while BM has to go 20 in MM for Mortal shots, imp. concussive and lethal shots.
0/41/20 has quite a lot of good things for PvP.