Diesel tends to be used more because they tend to be much lower maintenance than other fuel engines, and the cost of the fuel itself is also lower due to the higher efficiency.

I think also storing diesel is easier. It's not very flammable as compared petrol (gasoline) or LPG / LNG.

Although diesel can get microbial contamination if not used in 6 - 12 months.[1]

1. https://en.m.wikipedia.org/wiki/Microbial_contamination_of_d...

Why are they lower maintenance in this situation (i.e. power failsafe generators that are used for only a few hundred hours over their life)? I appreciate that diesel car/truck engines are lower maintenance, but they are used in a very different way.

Easier to generate a fixed-frequency AC output. Diesel engines can run at a fixed RPM and adjust fueling to match load, where gasoline and natural gas need a narrow AFR and have to vary rotation speed for power output. Maybe less of an issue these days as inverter/generators are more common, though I dunno if that's the case for the big generators.