It's not quite so simple as just adding more wards. Wards are generally a strain on server resources, because as long as a single person is in a ward, the game has to keep up an instance for that ward with like 30+ uniquely designed houses with lawns. They can only close the instance whenever it's completely empty, which in many cases isn't often, especially with FC houses. I know my FC's house almost always has at least one person idling in front of it.
Not to mention they are no doubt gearing to add a LOT of wards whenever Ishgard housing drops, assuming they plan to have equal amounts of Ishgard wards as every other ward.
I really do wish they had just gone for instanced houses instead of wards, they'd no doubt be able to have a much bigger allowance of houses, since it's much more likely for an individual house to be empty than an entire ward, so they can close instances more often.
And the instances thing really is the issue. For a non-housing example, look at Raubahn extreme. Tons of players trying to get into the same instance overloaded the server, making it impossible for anyone to.