Where's The Harm In Letting A Group Through?

Hello. I was very surprised today to see a group not considering to let another group through. Group A was a foursome, ahead of Group B, a threesome. Group A was many holes behind the group ahead of them. Group B was on Group A's tail.
Why not let Group B through? What's the harm? Is it an ego thing? A fear of not finishing (which shouldn't happen in the morning)? Or something else?
I finally had to tell Group A that I, in my role as marshall, would give them a yellow flag If they didn't let Group B through. They agreed, because they were on a par 3 looking for their tee shots. Group B was very thankful.
What will it take to get groups to let other faster groups through? I've thought in this case if Group B bought Group A 3 drinks, it would be a strong incentive. However, when I mentioned that to a player as an idea, he didn't like it, because he thinks letting people through is the right thing to do, and there should not be any incentive needed.
While I agree that's how it should be in an ideal world, we do not live in one. How else can letting groups through be encouraged? Please let me know your thoughts.

Regards,
Steve