I actually think it's to do with people not wanting to do the (perhaps stereotypical) thing of 'guy who spends all the time telling the world how macho he is always tops'. Also there is the fact that there is nothing nothing nothing that Dean wouldn't do for Sam.

I, er, will point you to this illuminating poll (http://sevenfists.livejournal.com/124341.html) and the ficlet inspired by it (http://intimations.org/fanfic/supernatural/options.html) by astolat.