Not accusing you of this, but the primary reason people run outdated software is a really problematic insistence, mostly by front-end people, on using the new-and-shiny instead of the tried-and-true. Breaking changes galore - so people stick with the past as much as possible.
Example: Office 365 OWA doesn't work well on modern browsers other than the latest version of Edge on Windows. But it does work fine on browsers that are older or pretend to be older! I'm technical enough to spoof my user agent, but Mom & Pop are just going to say "I don't like the new one, it broke stuff" and that will be that.
Pretty sure the primary reason people stayed stuck in IE for so long was that Microsoft shipped it by default until very, very recently (even in home editions of windows).
You can add on top of that the rebranding they went through. If they had just done what they did now, and shipped the old rendering engine inside the new one as some kind “legacy mode” that sites could request then everyone else could have moved forward years ago and not had to deal with a stagnant browser.
For me, and maybe this is just my organization, but the new O365 OWA interface is extremely slow. Latency is so bad that typing results in only 1 in every 4-5 keystrokes not failing to register.
If I pretend my user agent is from circa 2012-2013 (e.g., Internet Explorer 10), it will default to an older interface that has marginally fewer features and is much, much more performant.
Example: Office 365 OWA doesn't work well on modern browsers other than the latest version of Edge on Windows. But it does work fine on browsers that are older or pretend to be older! I'm technical enough to spoof my user agent, but Mom & Pop are just going to say "I don't like the new one, it broke stuff" and that will be that.