Always a lot of reasons, however when you get experienced you become better at guessing the reason for the cb.
In my case, asking or dancing with both of them right away instead of picking one increases the chance with about half, since;
Her friend won't be left alone while she's dancing.
The friend will have tried you too.
They might've not tried dancing with one guy at the same time and it's a bit exciting.
And ofc pre-open with EC + showing you got fun by dancing to the beat etc. else the chance is much lower even if you try to dance with both at the same time.
Learn some dance moves too. Wraps, windmill and spin etc., and then don't dance for too long because it will only make your attraction -that- high and we all know if we don't progress in the attraction and seduction it will fade.
My order of doing it is usually I'd dance near the girls or if there be many just dance with or without friends. Then I'll move over to the side of my target and talk over the shoulder asking how good she's at dancing or that she seems a good dancer or whatever. Or I'd simply hold my hand forth for her to take while having EC.
From there I'd do a spin with both or just my target (if it's both I'll do a few swings at the same time with myself under their arms and opposite and then start dancing solely with target leaving her friend to herself).
At most I'd dance for 2 mins. And if she's not up for moving somewhere else discard her and go for another girl on the df, since if you have good skills other girls would've noticed and be interested (social value through dancing skills).
The reason why GC don't cover much about dance-game is due to it being very ineffective and it's not their style. A lot of the girls out there are simply attention-seekers and it doesn't cover much of seduction. You will meet a higher rate of cbs on df and you will have more girls saying "no" when asking for a dance, however you can use the dance as pre-opener when you go over to her table later on.
