I'd say it's cultural. Stiff upper lip and all that. It would be impolite or even confrontational to complain about something, so you have a little moan instead.

A common response to the question "how are you?" is "could be worse". I've always felt this is a good example of British culture, though maybe it's not so simple to understand if you've not lived here.

A casual moan about the weather or the fact that the potholes only ever get haphazardly filled in a week before people need to re-justify their budgets for the following year is a great way to blow off a little steam while building social bonds with the people around you.

These mild inconveniences are perfect when filling dead air with a friend or passing the time with a stranger at the bus stop (it's always 5 bloody minutes late, why do they even bother with the digital signage if it's always wrong? Ah well, it is what it is).

As for causes, you'll get many varying answers, from Thatcher to that just being how it goes.

I believe it was Family Guy season 3 episode 4 in which the Drunken Clam is taken over by the British and turned into a pub, and when Peter and his pals try to storm it and take it back they get defeated by means of biting wit and clever sarcasm. That kind of focused negativity would be an awesome image to live up to, but instead you paint a picture here of habitual incoherent cattiness, it seems like some unfulfilled potential. But I have no specific proposal in mind, guess I'm just moaning about it.