One thing to note is that X-Wing is vastly different from Armada or Imperial Assault, games where you have slow moving grand capitol ships or a defined grid of spaces to move to. I'm sure warhammer doesn't force every one of your units to move forward at least one inch a turn.
It's easier to implement objectives in those games than it is for X-Wing. It wouldn't really work that well or make any sense thematically if capture the flag was tacked on to X-Wing for example. It wouldn't be that fun either, what would be good at the standard game mode would also likely be just as good at objectives.
Let me also add that one of the objectives in Armada- the one where you place all of your ships and then your opponent sets hyperspace spawn points behind all of your ships and spawns in whenever they want to in order to automatically win wasn't very fun. I'd rather just play deathmatch at that point.
It's just more fun generally to play the core game than to have a bunch of sprinkles and gummy bears and other crap on your ice cream. You can keep your stupid random items and stages where an Arwing strafes eveyone, I just want to play Final Destination because I'm playing the purest form of the game without any crap.
I have a strong suspicion that everyone who wants objectives in the game is bad at it and can't enjoy the game in its normal state, so in order for them to have fun they need their special snowflake game mode. Just like bad Smash Brothers players want to play with items and cuh-rayzee stages.