Why is New York-London used in time measurement for aircraft?
They are (and historically, were even more so) two of the biggest cities in the business world (ranked Alpha++). That specific route has some of the highest rates of business travel and is, therefore, a fairly prestigious, famous route which is popular with fairly wealthy, prestigious people.

They are also two of the most famous cities in the world, and most people know roughly where they are on a map. The fact they're separated by an ocean which is large but was just small enough to be possible to cross safely in the early days of aviation also helped, because it gives the impression of being further than it is.

For example it's a comparable distance to London-Dubai, but that doesn't "feel" as big due to being overland through Europe. Equally while many people know roughly where Dubai is nowadays, it wasn't anywhere near as famous 30-80 years ago.