Because for most use cases, Mint works flawlessly. It changes little from time to time. It has all the drivers to get started with a wide range of common hardware. It has all the codecs to play common media formats.
Of course if the package update is too slow, it’s not for you, but then unlike you, most people don’t need the latest and greatest. They just need something that works from the get-go with predictable behavior.
It usually has something to do with programming. Again, most cases, the versions in the packages included in your garden variety stable distros should cover most use cases.
However, once in a while one would encounter the need of using the cutting edge features on certain compiler or interpreter. Rust comes to mind. I know Python introduced some features that could drastically alter workflow (e.g. switch statement). NodeJS is another one known to be lagging behind from time to time.
In other cases, hardware support might be taken to consideration, especially for newer machines. However, with Mint including the optional newer kernel, it shouldn’t be a problem.