4chan is being actively bombed with interracial/trans porn in an effort to make it less palatable to rightoids. It really just makes it less useful and interesting for everyone but don't bother telling the spammers that, they're on a mission from the secular god known as hormone pills.
In my experience, people tend to drift towards whatever is the most taboo for their personal moral framework. I guarantee for people frequenting 4chan, that is trans porn. There is no other way. I could safely bet money in the triple digits.