Imagine the pvp realm with 1000 players.
500 players will not exist for a faction since the realm is not cross faction. That also means the global chat will look more empty and that can discourage people to play and try this realm.
Allowing a cross faction world chat could help to encourage people play in the pvp realm and not seeing it so empty
Also, it could help to easily create some "events" between both factions, between certain guilds, to globaly inform where a group of players are attacking others, which also could encourage players of both factions to go to that place to help!