The grid is built on the assumption of a few central points producing almost all electricity. This creates a grid focused on distributing away from those points. A more distributed and fluctuating producer situation will need more distribution between different branches of the network than the current infrastructure can handle. It will also require extra power generation that can be started on demand when the power production is lower (due to clouds, night, etc). This requires a lot of extra capacity that will get very low average utilization and therefore be very expensive due to being expensive to set up.