The reasons are likely more political than scientific. The rovers keep the money flowing. Rovers have a high chance of success and they are some of the last few

The reasons are likely more political than scientific. The rovers keep the money flowing. Rovers have a high chance of success and they are some of the last few remaining tangible results that NASA can point to to justify their budget in the public's eye. A new kind of mission is risky and would likely get bogged down in bureaucracy (see Orion).