Few factors - 1. Design goals. The program had a budget, and with that budget their goal was a 14 day target. Anything extra is icing on the cake. They could h

Few factors - 1. Design goals. The program had a budget, and with that budget their goal was a 14 day target. Anything extra is icing on the cake. They could have used an RTG to ensure long term survival, but it would have exploded their budget. 2. If I remember correctly the probe is in the south pole of the moon. Most probes are closer to the equator. The poles get colder and I think in some circumstances have more night time 3. Experience. This is their first successful moon probe. Now they understand the parameters better, so maybe next one will do better?