Perhaps the old one was leaking, or had a similar flaw?

Come to think of it, "my" postbox (round the corner, on Chesterton High Street) was changed recently; I saw them doing it, but it happened more or less same-day, I think.

Maybe they're aliens in disguise. After all, it's nearly time for a Xmas Dr Who episode .

Perhaps they thought no-one would notice.

Maybe they're replacing them with boxes that are somehow more secure? (harder to flip a lit cigarette into, or to get rain into, or to break into and steal things?)