Because, as UltimateE and I pointed out in the thread discussing the future of the Ultimates, there aren't many things they'll get trotted out for. Spidey has the IUGMs, the X-Men have their fellow mutants... so, outside of huge things like Galactus, Loki, or the Chitauri, what use would we have for them? Therefore, logically, if it's an Ultimate story, it's got to have something major, or else Spidey or Xavier's people would be dealing with it.
Volume 1 was half-setup, half-Chitauri. I expect Volume 2 to follow a similar format with either half-assorted, half-Loki, or half-assorted with Loki, half-something else big.
Considering that most people dislike team books where only two or three members show up, it's a given that everyone will be involved, and why would you need the full might of the Ultimates unless it was something big?
I think another possible role for the Ultimates would go along the lines of the conversation between Cap and Fury at the end of Ultimate Six. Cap mentions the next war to be fought will be a genetic war. The Ultimate universe I think is capable of performing "operations" or experiments to enhance normal human beings and giving them powers. We could see the Ultimates deal with this genetic war possibly happening sooner rather than later. And they could introduce new characters to perform the covert ops that would include other countries getting their hands dirty with such experiments, since the Ultimates has become a public team in Vol. 1.
I believe Scarlet Witch and Quicksilver are already on the existing cover ops team run by SHIELD. Besides, since such a war would drastically affect every aspect of the Ultimate universe and preclude any further storytelling for a while, I doubt we'll be seeing the "genetic war" soon.
Well, look at it like this:
The Ultimates are the official US-government superhuman response team. They'll get called out every time the US government wants to make sure something is done right. They would get calles out if Kang showed up and declared war on the US. They'd get calles out if Magneto or any other major villain ever escaped from prison. They'll even get called out if the US were threatened by conventional threats (war for example). There's enough for them to do, believe me.
And unless they want to turn Ultimates into 616, you don't need an ongoing title. "Oh no, Magneto escaped... again!" How many huge threats can you pack into a rapid enough time span to make an ongoing feasible, without it getting ridiculous? And a war just simply isn't a cost-effective use of the Ultimate property, because it'd tie up all your characters, any preclude any further storytelling of individual comics until the war was taken care of.