Many organizations are stuck on Java 8 because Java 9 made breaking changes. (Java 11 also made breaking changes.) Many of these changes also broke some popular libraries at the time that hacked into the JDK internals, so it meant a significant refactor for software that might be in maintenance mode already.
To clarify Java made changes to things they never promised would be a part of backward compatibility. Libraries were using things from JDK internals/implementation details, things that were never guaranteed to be there.
It isn't that severe of a break. Most of the friction was with the libraries with transitive dependencies, like ASM, that depend on a particular bytecode version, using internal classes that were removed (e.g. sun.misc.BASE64Encoder), or the new module system encapsulating APIs that were intended to be internal-only (usually fixable with JVM runtime flags.)
A lot of organizations that use Java are very conservative with their tech changes, and didn't see a compelling reason to upgrade. This was especially true since Android Java still used Java 8, so many popular libraries continued to support Java 8 for their Android userbase.
