It’s really difficult to beat diesel for many reasons, sure solar + batteries is with enough for though. But what if you need to augment beacause some were damaged in a weather event, IR dropped us a secondary genset next day after a tornado (mind you precontracted). Other fuel options exist but you can’t rely on piped natural gas in a highly active seismic zone (as they shut off entire areas after an earthquake to limit leaks). Diesel is stable for long periods, if it’s inundated with washer can be mechanically separated, then you add on the safety aspect of it being less ‘energetic’ than gasoline.

Not saying there aren’t other options, just the one most businesses have opted for, and has the best ‘vendor’ support (plenty of fuel companies to deliver, rental agencies to arrange the delivery, and companies that Veratis that setup the building side needs)