it's not cities or large towns in general; it's mainly bloody London. Glaswegians and Mancunians and Durhamites and Yorkers and Belfasters (Belfastians? Belfishers?) will happily chat with each other.
Cmon. This is about chatting on the tube - the main reason we don't talk on the tube is that it is almost fucking impossible to do so. When I get on the tube with friends, often we don't talk on the tube because you have to get intimately close to each other or shout extremely loudly.
Most of the times that we regularly take the tube (eg, for commuting), it's so crowded that the only thing you can get close to is someone else's armpit, because you are crammed in like sardines.
It's unpleasantly warm in summer; I wear separate travelling clothes of shorts and t-shirt and then get changed in to work gear afterwards otherwise I get to work dripping with sweat.
Pretty much the only way to cope with that crowdedness, noise and heat is to zone out and pretend you are somewhere, anywhere else. Music helps. Reading helps. Having an awkward shouted conversation - even with a friend, let alone some random stranger - does not.
Other cities in the UK simply aren't comparable. People don't really talk on the Paris Metro either.