Its because traditional nuclear plants are far too expensive and take far too long to build. By the time we were to finish building new ones renewables would have already been up and running for years at a far cheaper cost. The main issue is that battery and power storage technology isn't good enough to meet demand for nighttime hours when wind and solar outputs are at their lowest. This is where nuclear power would be useful. This modular reactors are much smaller meaning they are easier to spin up and cost far less than traditional reactor technology.