I think a big part of it, now that I've put some thought into it is that no one thinks (including myself) that this team is 'for real.' Its lost a piece of its starting rotation, its going to need a ton of young guys to show up sometime in order to do something, I think fans here have been so jerked around by a lousy product for the last few years that they don't trust the Dbacks yet. Hopefully with steady, consistent winning, that trust can be rebuilt.
And like I said before, the very nature of the Valleys geography hurts all sports teams, people who buy some cheap house out in Surprise don't want to spend nearly 2 hours a night going to and from a game. Phoenix is growing more dense however, I think the more condo towers, hotels, and infill projects we see built in the center city, not only will our sporting attendance improve, but our quality of life in general.