So MAN which is comparable in size to LGW (N) has an electrical demand in the region of 20 to 30 MVA.
The largest portable diesel that is fitted in a 40ft container is 1.6MVA and there are not that many sat in hire yards around the country available for immediate use, needs a separate external fuel tank and uses about 500gal of fuel per day, so do the sums. (A 100kVA unit with fuel for 18hrs weighs about 2.5T including a 2 axle trailer. A 200kVA unit is the largest with fuel as above that can be mounted on a 7.5T pick up, I know as we have some at work and the trouble that was taken to keep it within that limit)
But how often does a failure of this magnitude occur?, Is it financially viable to have back-up power to continue full operation? (look up the meaning of reasonably practicable)
OK in a hospital where there is risk to life (and even they do not have back-up generation to continue full operation and often for not more than a few hours owing to fuel storage issues) you need to have something robust in place. But the result at an airport is no where near that, as long as safety related equipment is kept operational, and sufficient lighting, what is the justification to maintain full operation - just so folk can go on holiday!
Put it this way, how many would have a 2nd car on standby to cover for the eventuality of the first one breaking down on the way to an airport (to go on holiday)?
But of course it could be done, at a large expense so increasing charges to the airlines, which of course they will recover from their passengers. Which means airfares will rise.
Sorry this is not a "no risk" world, things go wrong when we don't want them to, but they will despite the best endeavours of people