Because it's not that simple. If you want 100% availability year-round then you need about 2X overproduction and quite a lot of storage, not just the four hours normally paired with solar today. That could end up being more expensive than nuclear.

But that doesn't change the fact that solar on the margins, without the availability requirement, is quite a bit cheaper than nuclear. So going 100% nuclear probably isn't the cheapest option either. The optimum is a mix in the middle somewhere.