Solar and wind are simple and very amenable to mass production. Many parts of them are usable for other purposes -- we need generators for things other than windmills, inverters for things other than solar powerplants, etc. This means we're not making things as a one off, but doing mass manufacturing, and the different users all reap benefits.
They're easy to produce, which means there's lots and lots of competition, which pushes down price.
They're easy to install, scale and maintain.
They're easy to iterate, because it doesn't take billions to test a new design on a small scale.
They don't need most of the parts nuclear has -- if you think of it a windmill contains parts a nuclear plant needs too, but unlike nuclear doesn't have all the nuclear stuff along with it, which isn't cheap.
The way I see it, nuclear could have been successful but is going the way of the mainframe -- today mass computing is done on huge amounts of commodity hardware, and in the same way large, purpose built plants are already being overtaken by mass production of panel after panel being churned out of a factory, and that's not going to get any better for nuclear.