The reasons are:

- Running billions of dollars worth of GPUs only 30-50% of the time insanely wasteful. Everyone else is running them at 99% duty cycle or close to it.

- Solar is big and generally has to be built far from where the data centres are. Transmission is already a bottleneck in many states. Adding tens of gigawatts requires non-trivial large scale infrastructure investment by… the utility companies, not SpaceX. Not to mention regulatory hurdles, etc.

- Battery production is already mostly maxed out, so obtaining tens of gigawatt hours of capacity on short notice is a decent percentage of global production.

Gas and other fossil fuel power plants can be deployed adjacent to the data centres for effectively zero transmission loss and no long powerlines that upset the NIMBY types.

They can work 24/7 and are largely available right now, but as others have pointed out the turbine blades are a limiting factor with these too.