For centralisation - large areas of the grid are dependent on a few locations, so if there is an issue with one or two areas, the entire network can fail. Say for example if there is an earthquake which disables two nuclear power plants, that could cause massive issues with the grid.
If you have many small power sources distributed across a larger area, it significantly mitigates the issue - the loss of even dozens or hundreds of wind turbines would be able to be handled much more responsively.
Nuclear is uniquely disadvantaged at having very bad responsiveness to demand. Renewables are extremely good at that, coincidentally.
For security, I’m sure you can imagine many scenarios, but nuclear waste is a potential target for creating dirty bombs for example.