There is the possibility of being intimidated and thinking that the better dancer may not enjoy him/herself as much with you.
I got upset by one instructor who, when asked to dance in this manner: "I'm just a beginner but I was hoping you'd dance with me anyway", shook her head and said no.
I swear if I'd run into an instructor who turned me down after I asked like that, i'd have left the dance floor and never come back.
(Yes, yes, instructor has the right to enjoy himself at a dance, but still.)
Of course, given my rate of progress in improving so far, that may have been a relief to the other dancers.