Because (1) they are at or near capacity, (2) they are very costly to operate so those 'reasonable rates' are very hard to come by when using nuclear as power source. There are other, cheaper sources for power than nuclear that are more in line with what data center operators are willing to pay. There has been some talk of data centers with their own power plants. But usually that centers on fossil fuels rather than on nuclear.

There are some efforts in that direction though:

https://neutronbytes.com/2025/05/07/google-plans-three-600-m...

And I'm aware of a much smaller scale project in Europe besides this one and there are probably others.