Bear in mind, all the expansions for the game so far have supported play on a 3x3 ft surface at 100 points. Players are of course free to experiment, making up their own customs and expansions. So far though, FFG has not made a thing that breaks the standard tournament play environment.
Capital ships, if they are introduced at scale with the other miniatures, will break out of the setup area and so be illegal according to the rules of standard matches. They would then have to be something for special cases. On top of being a minature at drastically higher prices than $30 (and so become a not-casual purchase, which so far casual purchasing prices I think has helped X-Wing explode into popularity).
So ask yourself; are you willing to casually spend around $80-$90 USD on an expansion that you can only use for a handful of special case missions?
I have yet to see anyone play missions from the rulebook. Every X-Wing match I've seen has been a pickup points-based match. If this even remotely represents the player base for X-Wing, nobody is going to want to buy a capital ship under these restrictions.
Sure, there are going to be some, but I don't think players casual than we are (who don't go to the forums, and who don't spend lots of money on creating specific ships) are going to spend money on such a product. And really, FFG is trying to appeal to everyone, including those gamers who don't buy into the game enough to run lots of copies of the ships.
Yes, it's FFG's decision of course and they might surprise and amaze. But from the perspective of the non-capital sayers, we don't see why FFG would do this when there are logical reasons why they shouldn't. I mean, can a reason other than semantics be given for why Capital ships should be in the game?
Personally I'd rather FFG focus on getting more fighters from the expanded universe into our hands. We seem to be doing fine enough in making our own capital ships and stations ourselves for our own tournament groups.