Cuz for snipers orbital strike is mostly defensive skill i.e. cast on himself! And now its pointless...
Try to 2x snipe and Ambush without an OS without an OS and still be usefull to your team in an arena if you are getting focused as an MM. It makes the loss of life more of a trade of for the attackers. Along with giving other ranged protection. Even if everybody was smart and stayed out of it I could still determine where the battle took place. In arenas you can also destroy a teams ability to focus. MM is one of the few casting classes in arenas that can loose damage because of LOS, but still provide noticeable support for their team. Lethality can still have good numbers, but are they as usefull and are still easier to take down without the CC immunity.
For MM in PVP and PVE this is basically removing a top level ability that had realistic issues nobody was complaining about. Corrosive dart ticks on 2 other people in an arena are not enough of a trade off for the MM spec, but they do help the Lethality spec that was said to be the reason for the nerf.
Hopefully this is all politcal so they can scale it back and people will be happy. That would at least show the use of logic in why the nerf is the way it is now.