Because solar panels powerful enough to run a drone are large and heavy, and/or fragile. A solar panel sufficiently lightweight to be lifted by a low power drone, and simultaneously powerful enough to supply that drone, isn’t easy

It’s a very capitalist way of thinking about the problem, but what “negative prices” actually means in this case is that the grid is over-energised. That’s a genuine engineering issue which would take considerable effort to deal with without exploding transformers or setting fire to power stations