testing Beta versions takes time and effort. Most companies are reluctant to let their IT staff spend a lot of time on testing Beta versions. There is nothing productive in this testing for the companies.
When a user group does acceptance testing on a new system, they have a definite interest because they are going to use the system to do their work. This is the big difference. If they don't like something in the system, it will be changed, else they will not allow it to get into production. On the otherhand, when Beta testers don't like something, it's just their opinion, and may or may not ever get changed or fixed.