I guess because survival is a pretty decent spec outside of pvp and even in pvp you can make it work. Homerjay was playing it on stream and he even said that dmg can top MM if you can lock and load at the right moment.
I mean, combat rogue and even more, holypriests (who really have a hard time in PVE and PvP) or blood DKs probably asking this question for years but they never got an answer.
Imagine having silence shot as SV in pvp. You can literally slay stuff in a matter of few seconds once LaL procs. This would be kinda broken. Black Arrow is extremly situational, even on single target (PVE) explosive trap + glyph will grants you mostly more constant LaL procs and deals more dmg. Try to use it in pvp aswell. Its a nice burst on proc and If you can pin the target on it you will be surprised how strong it is.