Individual states could do that now. Notice that the major tax incurred by a data center is state/local property tax. It's why they often go looking for places with a property tax break, so giving it to them on the condition of "no fossil fuels" could be a pretty viable strategy.

There have often been private deals between renewables and big energy customers but the finances don't always work out how you might expect. Renewable projects are built using debt with interest rates largely based on the likelihood of generating revenue by selling electricity. And because of the long time scale interest can snowball to form a substantial part of the cost. Having a guaranteed customer in the form of a data centre is balanced by the risk that they might go out of business. You are basically coupling the risk of the generator to the risk of the consumer. That is why CfD style schemes are often favoured because it is just a safer source of revenue. Obviously hyper scalers have access to very cheap credit, but they may end up paying more for the electricity.