Often you'll see people argue that excessive regulation is what is driving costs up, but without a very high level of maintenance, nuclear power systems will inevitably have an unacceptable rate of failure. These safety margins need to be higher than those for units like oil refineries, coal-fired power plants etc., as the consequences of failure are so long-lasting (fires at oil refineries are relatively common, for example, but they don't result in 100-year no-go radioactive exclusion zones).
Therefore, to make nuclear accidents highly unlikely, maintenance costs amd engineering standards will inevitably be much greater than in any other power supply system. This means nuclear will always cost significantly more than any other system - and this also applies to storage of too-hot-to-handle 'spent' nuclear fuel, as well as to the costs of eventual decommission. In addition, the nuclear fuel supply chain (high grade uranium ore -> fuel rods) is fairly complex, and subject to geopolitical and economic price spikes.
All in all why would a utility manager these days looking at all the options for non-fossil-fuel power go with nuclear?