In the United States, I think that permitting, interconnection queue backlogs [1], and political efforts to protect legacy coal against cheaper competition [2] are the biggest obstacles.

That said, the US had its best-ever year for solar deployment in 2023, and this year is projected to be even better: https://www.eia.gov/todayinenergy/detail.php?id=61424

[1] https://www.utilitydive.com/news/grid-interconnection-queue-...

[2] https://montanafreepress.org/2023/06/03/the-battle-for-clean...

It's not a replacement for the cost of operating existing gas and coal plants; building up new solar capacity enables you to burn less gas or coal at the times when the sun is shining and thus cut its fuel costs, but it does not enable you to shut down the plant and cut any its other operating costs, because you still need the same capacity for e.g. winter evenings when at peak daily consumption there is zero sunlight.