"Simplificate and build in lightness."

This is an expansion on the old KISS principle because a simple system isn't necessarily a light one. Simplicity and lightness go well together in all types of aerospace vehicles.

Bill Lear, when he was directing the development of his first Model 23 Learjet, directed his engineers to keep all the systems simple and light. He told them that any component that isn't absolutely needed should be left out of the design. Not only do you save the weight of the component, you save the weight of the backup system.

Using solar arrays for a manned lunar spacecraft has some advantages but not many. Solar arrays would be very useful on the lunar surface for extended stays, but you'd still probably want some form of nuclear power for the 2 weeks long lunar night. During the flight to the moon, solar arrays might just end up being in the way. Not only would you need the mass of the arrays themselves and the associated deployment & rotation hardware, you'd still need some backup capability. Fuel cells have the advantages of being pretty simple, well proven, compact, and providing drinking water as an operational byproduct.