The issue with PVE content is that it takes a long time to develop and tune properly, while PVP is relatively dynamic and self-regulating. Obviously new releases need to be balanced against what already exists, but that probably isn't as daunting as figuring out the exact stats and abilities to give a new raid boss to ensure it's extremely challenging while still being beatable.
Personally I do prefer raids to PVP, but Grand Arena is actually my favorite addition to the game. It rewards building multiple strong teams, rather than the "sink all your best mods into one team and ignore everything else" methodology that made sense when Squad Arena was the only game in town.