I chalk it up to lack of budget
Always the case on this show.
the Daxamites should have spread out and attacked multiple cities.
Thats ounds far too logical for the way this was written. Everything has to be aout that one city. Nowhere else. There have been endless Superman comics over the decades where a crisis hit places other than Metropolis, and in the MCU, battles (with potentially far reaching consequences) all did not take place in one city. The problem is that in order to make SG appear to be the center of the universe, all things have to happen in that one city, when she would appear to be a far bigger protector if she's regularly racing around the world fighting threats.