Because we could use the money spent on nuclear to build more renewables and supporting infra (storage and transmission) than if we also built nuclear. The renewables will snap be finished and replacing the fossil fuels a lot sooner than the 10-15 years for a nuclear reactor.
If you look up studies into it you need a lot less storage than you'd expect to run a fully renewable grid, as the scale of the grid stabilises it to weather fluctuations. Winter also is a problem that can be overcome. That gencost report is a decent starting point, there are plenty of other studies into it though. The low cost of storage is also especially true if you're looking at the first 99% of the grid.
Maybe those studies are wrong and nuclear would be economic for that last 1%. However, if we can get to 99% years earlier by just building renewables then discover that it's harder than expected to get to 100 (somewhat unlikely, especially as more storage tech is developed), we can build nuclear then. The net carbon from getting off the majority of fossil fuels years earlier will probably make it the better decision anyway.
Also just noting that my views are based on what I've read about Australia so you should also find peoperly researched cost analysis for your country. Also for renewables to work well in smaller countries they'll need to develop more interconnects their neighbours etc.
