System design derives from mission and project design. The lander is a demonstrator for a future rover, they had (for this particular mission) no intention to manage and support a lander for a substantial period of time. They basically bolted a few science instruments that fit the weight and power budget on the frame – but they are in no means essential to the mission.

solar panels in mars did not turn out to be very efficient. After couple of days, all those panels get covered with dust, and you need a wiper to clean that up, which will again drain a lots of battery power.