Because you said PvP servers didn’t have it, which is objectively false. If you meant that PvP servers didn’t have sharding prior to sharding being in the game, that’s a worthless claim, on par with saying PvP servers didn’t have war mode.
I recall rather well how much the servers would strain from a 40+ man raid fight outdoors in the Maw every Tuesday, so I’d have to disagree there. Sharding and Cross-realm enables them to better tune the number of people, preventing zones from being overcrowded or empty. I’d say that the Jade Forest being the only entry zone to MoP is a more likely reason for them implementing that, as it would have been even worse in WoD with everyone arriving at the same area going through the dark portal. Regardless, talking about sharding as a negative of war mode is meaningless, sharding happens on all servers to a degree, it is part of the game since WoD launched. It was on PvP servers before the War Mode option was introduced, and it would be there if they did add a PvP server again.