It's hard to beat the immediacy of trucking in turbines. Framing this as an either/or leads to bikeshedding. Why not allow them the now solution and mandate a better intermediate-term relacement?
If in a year Mississippi said "Plants are ready to deliver on that power purchase agreement you signed, at a lower cost than onsite turbines" then they'd be turned off.
Yet instead of a PPA, xAI contracted with TVA for wholesale power, is building their own dedicated turbines, and is using mobile turbines temporarily.
None of that helps the regional electricity market, because it's not integrated.
IMO data centers themselves are environmentally unobjectionable, but gas turbines are loud polluters and it should be almost impossible to get permits for more than a few months at a time.
The existing law could be used to stop them just fine but Trump has no interest in that.