As a brief report from Resources for the Future think tank explains:
"Renewable energy sources do not require fuel inputs to run since they use energy from the sun, wind, and other natural sources. Consequently, they are able to offer bids of $0 into the energy and capacity markets. As these sources make up a larger portion of the grid over time, these $0 bids can significantly reduce wholesale prices for energy and capacity and could discourage long-term investment for all resources. As a result, wholesale markets may need to adapt in the future to better accommodate different types of resources."
In other words, solar power that is eventually too cheap to meter creates the problem of how to pay for the infrastructure required to make and transmit.