They did not keep the show going long enough or film earlier, Princess graduating and then getting a job or getting married?

It seems that we do not really see Princess grow up and move on. Her character did not grow enough, in my opinion. :p

RY pulled the plug on the show, and due to its ratings CBS would have probably wanted it to go on for at least another season. As it was they reran it in prime time for another season.

The show had a chance to have her become engaged to Ralph, who tried to propose to open season 5. In the 6th season they could have developed a real romance for her (maybe bring back the A. F. jet pilot from season 3) but they chose not to. That could have been several scripts in a seventh season.

It's possible they discussed having her graduate in the final episode, but instead chose to rerun her high school graduation in recut form.