I'm no expert but here's my take.
I'd say that one large factor could be a failure to ask guys to dance and realize ownership of the issue.
Another could be poor positioning in the room (far from dance floor), objects blocking path to position or objects between position and dance floor.
Grooming and dress code (and ordinary social indicators) can be a problem too.
I know I'm gonna draw fire for this one but feminine dress and behaviour can help. Men are masculine and women are feminine (opinioin alert). Not everyone will agree here I realize as everyone has different preferences for company but this angle might be more popular. I've danced with ladies (instructors) who lead (and learned the followers' part) and that's pretty fun and enlightening. I've even danced with other men (instructors) and find it constructive so don't take me as a sexual biggot.
As a guy, I will dance with anyone but I have preferences and given choice I'll excercise them much of the time. Here they are in no particular order.
1) Persistant attendance at dance activities (practice and development of circles of people).
2) Takes lessons.
3) Fitness and agility.
4) Lifestyle: No smoking, no excess drinking.
5) Contact information. Not so obvious but ladies who I exchange contact info are better because I can arrange to meet them and practice or take lessons. I also figure that reluctance to exchange information (after reasonable time dancing) is a sign of dysfunction or a negative outlook (general distrust or involvement in bad social circles).
6) Open. No evidence of clique behaviour or hoarding of partners.
7) Preparation. Shoes that work. Not too much scents.
Note that none of these things are to do with the level of the lady. I like dancing at my level about 1/2 the time or more but I split the rest of my time for above and below my level.
Darcy