Cause no one knew what the signal was till the Doctor showed up and decoded it. All that was known is that the signal caused fear in all the races that heard it. Once the Doctor decoded it, he was there at it, they couldn't leave in case he brought Gallifrey back.
I mean why doesn't the doctor fly off? The time lords didn't seem to be coming through without an answer from him, so if he leaves, they stay put.
Unless the Daleks etc were so freaked out they were going to flatten the place anyway, which is possible, but I felt the explanations were rather rushed.