Cleaning the panels periodically on that size farm means continuous maintenance, especially in the desert, where most people seem to want to put them. If you clean them with water, that's going to have to come from somewhere (I'm not sure if high pressure air would be good enough). In utility terms 25 years isn't that long. Nuclear plants are good for ~80 years, so you'd need to 2-3x the capital costs right off the bat. I'm not under the impression we have storage where it needs to be either, so that's additional cost + infrastructure that's already baked into nuclear.

Grading the land destroys the existing ecosystem, and putting panels on top of the ground will obviously have an impact on that area of the biome. Deserts are important ecosystems, and if we thoughtlessly destroy them to build solar farms, we'll have to deal with the consequences 30 years down the road. I'm not against solar farms, I'm just not religious about them. They have downsides and there is a lot of money to be made, so it's worth being weary of magic bullets.