The series did not cover Nevada, Arizona and New Mexico (collectively "The Southwest") nor Alaska, and Hawaii. Volume 8 was going to be Canada; it was announced in Roleplayer, then withdrawn and was never released.
It's unknown why the series ended, though it is speculated that poor sales did it in, as many of these books are still available in their original form (i.e., non-pdf) from the Steve Jackson web site today.