It's also a matter of federal budgeting; when the rovers landed, NASA definitely did not have enough funds budgeted to keep tasking the rovers for 8,000 days. But as the rovers continued to operate, NASA requested more funds to continue, and Congress provided them.

Once a robotic mission accomplishes its primary objective, NASA has a great story to tell about marginal cost vs return of additional funds. It's not that uncommon; heck they still get funding to keep the Voyager program operating.