I'm not sexually attracted to penises. They don't arouse me at all, and actually thinking about it rationally they look a little disgusting (so do vaginas). However, the logistics of penises and their appendage-like properties allow certain actions and movements like blowjobs and thrusting to be easily and more effectively depicted in the 2D form. Mainly because it is easy to draw 'floating penises'.
Things like cunnilingus of the vagina and scissoring, which are the main ways yuri love can be consummated, are hard to draw and floating vaginas are not as ubiquitious as a imagery technique for people to be able to enjoy their hentai without being distracted by it. To me, it is not as sexy. It is a turnoff seeing men in my pictures and doujins and given the logistical difficulties in two-vagina sex a common compromise is to have futa-on-girl pornography.
However, girl-on-girl yuri still be novel and intensely arousing if it manages to pull-off psychological techniques effectively. I can't fully describe 'psychological techniques' but it could range from a domination-submission relationship to implied connotations in speech. It is difficult to do this because it requires not just skill in visual imagery on the author's behalf, but good writing. Because of it is rare and drowned out amongst the battalions of simplistic jack-rabbit sex hentai, coming across one example of this is new and exciting.