The last try at a modern reactor in the US started around 2009, was supposed to cost $14 billion, and was supposed to be operational 7 years later in 2016.

Instead, it wound up costing over $30 billion and just now became operational.

In other words, it was a total shit show. No one likes to risk that much time and money very easily.

Smaller and cheaper reactors that shouldn't take decades to build are a lot more palatable to the people putting up the money.