I think it might be because it is more 'acceptable' to be seen in public wearing a bikini because it's designed to be worn on the beach or at the pool, in front of other people.
Underwear is used, obviously, to be worn underneath the clothes and it isn't a normal occurrence for any member of the public to see you in your underwear, so girl's feel less comfortable wearing it in front of people.
It seems a bit silly as they all show the same, but it's just what each one was designed to do that makes the difference, in my opinion.