Because total load on the power grid peaks at 8 PM when solar is providing nearly 0% of the power [1].
This means that no matter how many additional solar panels you add to the system, you are not displacing a single fossil fuel plant. Sure it's good that those plants are running nominally fewer hours per day per additional solar panel, but the cost to own, operate, and maintain the panels is roughly constant.
This results in the effect of increased prices for fossil power in a way that can't be offset by solar.
This means that no matter how many additional solar panels you add to the system, you are not displacing a single fossil fuel plant. Sure it's good that those plants are running nominally fewer hours per day per additional solar panel, but the cost to own, operate, and maintain the panels is roughly constant.
This results in the effect of increased prices for fossil power in a way that can't be offset by solar.
[1] https://en.wikipedia.org/wiki/Duck_curve