I believe CoC got its start on an old forum called futanari palace back when it was called unnamed text game. Then it gained popularity through 4chan's /d/ which used to be like 90% futa. The western erotic games scene used to be really small. The mainstream stuff was found on newgrounds, but that was mostly people tracing Japanese eroge. For everything else, your choices were pretty much limited to tfgames, legend of krystal forums, AIF and a smaller niche communities like futanari palace. The ones that were actually popular, like slavemaker or CoC, found their way to 4chan and blossomed from there. So when you have a popular game in a small scene, you're going to attract more people who are into that niche and also create people who copy that to appeal to that popular game. The predictable result might seem like an over representation of said niche.
Theres also the fact that due to how CoC1 works, you were generally better off having both set of genitals since that increased the amount of scenes & options you had. Having a lot of people playing futa characters means a lot of the submissions are also going to feature futa NPCs.