It could be because it's illegal, weird, or you just don't like it very much. We've had two questions about ships we like, I want to know what ships you all hate. Because I hate a lot of them. A lot of them.
I personally really hate soukoku, 60% comes from my hate for Chuuya the other 10% comes from just how much I've come to dislike Dazai... and the remaining 30% is something that'll take up an entire hour to type out but bottles down to they-are-not-compatible-long-term-at-all-unless-you-ooc-them.