My intuition is that it's a function of how many people are doing the type of work you're doing and how fast the environment within which your code runs changes

My intuition is that it's a function of how many people are doing the type of work you're doing and how fast the environment within which your code runs changes. Front-end JS is where both of these factor are at their peak - it's often the first stack new developers use, and there are a lot of devs doing it. Add in the speed with which the browser landscape changes and the (relatively) recent expansion of JS onto the server ... and everywhere else ... and you kind of have a perfect storm. The other extreme is probably something like legacy systems development - like maintaining and expanding banking and financial systems - or embedded environments where the hardware has a long life expectancy and is not easily upgradable.