Earlier quoted context omitted.
.. just to piggy back on this and add a "why": the smaller long range planes are allowing airlines to connect smaller cities with direct flights while still making money. This is easier for customers, and is insanely popular. An example here is British Airways' service from New Orleans to London. Obviously, some amount of people have always wanted to get from New Orleans to London, but previously they've been flying…
But New Orleans/London isn't two smaller cities, it's one smaller city and one major intercontinental hub. Doesn't expecting to have a nonstop flight from London to every city in the world put unsustainable pressure on landing slots in London?
When I was a kid and lived in Santa Cruz, we would always have to drive to SFO for international flights because SJC's runway was too short for some of the bigger jumbos. Now with 350s and 787s, its not an issue.