I have a friend who knows Robin. Here's the real scoop.

Hometime offered to renew her contract for the 13th season last
August-04. Throughout 2004, Dean and Robin participated as
spokespersons for Little Giant Ladder and shot an infomercial
demonstrating the product. Within months, the infomercial was ranked
among the top 10 in the country.

Little Giant Ladder replaced Dean with Richard Karn (Al from the sitcom
Home Improvement) and asked Robin to continue as a spokesperson and
shoot a new infomercial with Richard. As I understand it, because of
her contract with Hometime, she had to leave the show in order to take
the deal with Little Giant.

Apparently the deal was too good to pass up. So she declined the offer
to renew with Hometime for a 13th season.