Mojave

Coverage today in a number of newspapers about Feinstein's effort to preserve the Mojave interfering with turning the desert into a giant solar collector.  The energy side complains that it would remove a great location for solar power.  These stories don't explain why solar collection needs a big desert.  There is ample space in the places where the energy is needed.  Los Angeles, for example, seems to have plenty of rooftop space for solar collection.  I assume there is some technological impediment to using all those empty rooftops.  Or is it legal?