The reason for this is that the military is very experienced with building the smaller reactors. It also doesn't hurt that the reactors are remarkably small since they only need to power a single ship or sub.

There's also the reality that the military doesn't have to worry about expenses. They spend what needs to be spent.

In the civilian world, however, they're trying to build 'small' reactors that really aren't that small, and they keep trying to pitch them as 'cheap' to build. They're not.

Why? Because they need funding. That's it. Nuclear reactors, in the US, need decades of time to be built and decades more time before investors will see a ROI. That's a hard sell for anyone, so they pitch them as being cheaper than a regular plant. They undersell the costs and then start to come clean after a few years.

Could it be done quick and cheap? No. Time, money, quality. You get to choose two out of these three. You want quick with quality? That's going to cost you a ton of money. Quick and cheap? Low quality. Quality, with a cheap cost? That's going to take a fuckton of time.