We personally struggle to just keep Ubuntu updated with LTS releases, let alone managing all the updates to ensure software works on the new packages installed on whatever LTS version.
The issue is that when you have smaller companies with small web development departments you end up with a lot of differents sites/tools made over time. So now when it comes upgrade time you have a large pile of stuff that needs upgrading. One developer making new websites over a few years time isn't that crazy, so unfortunately you end up with a build-up of projects to update.
Yet while having time pressures to update, you also have pressures to continue to work on new things or fixing problems users have reported.
It usually comes down to time/money. Unfortunately dedicating time to updating things is never as high of a priority as making new things, adding new features, or fixing bugs. Because it follows the traditional mindset of it's working, why update it.