User blog comment:AYET/Chapter 748 - Talk about a debt repayment/@comment-24659186-20140521184901/@comment-4754162-20140522025919

Taking a guess, I think because of the number of significant adversaries at Doflamingo's end is limitted, thus Oda made the group split into two to avoid overwhelm the Strawhat matchups against the Don pirates.

Lets take a look at the lineups at both sides (significant characters with high level, superior battle prowess):

The Donquixote pirates: Doflamingo, Pica, Diamante, Trebol (and perhaps Issho siding with them).

The Strawhats: Luffy, Zoro, Usopp (KO), Robin, Franky

Then we have Law who should side the Strawhats.

Then there's Kinemon and his friend Kanjuro - though very not sure if they are really top class fighters.

There's also this Kyros guy

And not forgetting Sabo and gang

The rest (e.g. colosseum combatants, other Don pirate subordinates, Bastillem etc) I'd consider secondary supporting cast that I will not include into the equation.

Thus I think why Oda deliberately take away Sanji and 3 others from this and put them into another scene.