Another comment explained this. They set a goal of 90 days to get the data they needed. When they presented the proposal for budgetary approval, 90 days of data collection was considered worthwhile enough to greenlight the project. So they accomplished it. However once you already have a working rover on the surface you might as well operate it as long as it lasts because the marginal costs are minimal compared to the initial costs. So they kept requesting additional funding to keep the program running.

I see nothing wrong with this.