Regarding why CBS axed it, I think the show's time had just come.

It was probably too expensive to justify continuing. Ten years is a good run. They can already make good money in syndication and DVD sales.

I'm guessing that they bent over backward to get Alex to return for the past two seasons, and for Scott to return half-time (which is what he did). It probably wasn't worth it to them anymore to continue to push for this.