Some reasons I can think of:
- Bugs fixed never effected client therefore it's not worth updating.
- None of the features introduced are worth updating for because they were never heavily used.
- Perhaps new update introduces a bug that makes the update a non-starter and they won't update until a new official update is released that fixes it.
- The organization has a very strict process when it comes to updates therefore delaying updates by weeks/months on software.
- The organization is actively looking at other help desk software alternatives and therefore does not want to update the current software.
- The organization lost their "IT" person and therefore is not in a position to conduct an update for something that is currently working. I can only imagine this would affect small businesses.