[Comartemis]

I think the issue is that these doujins have a largely male fanbase, and a guy will... um... relate better, I guess, to the sensations involved with girl/futa than with girl/girl. As if the visual content wasn't supposed to be stimulating enough on its' own or something.

Go figure, I'll take two girls with a collection of vibrators over one girl and a futa any day of the week.

*EDIT*
Had another thought about this; straight sex demands that one of the characters be male, which lends the girl a feeling of being "some other guy's catch" to the reader. Futa is a way of having "straight" sex between two girls, thus the depiction of a relationship without the interfering male. There's also the fact that some guys just don't like to see too much of the male half of the equation in their porno (I know this is the reason I don't watch most real stuff), so two girls (one of whom is packing heat) is more aesthetically appealing than a guy and a girl.