Because subtitle language is the language you are trying to learn, the language spoken in the video, and translation language is the language that is assumed you already know. If you just flip English and Japanese around you will see the option.
In theory though, a native Japanese person, like a child that still has not learned all the Japanese characters, but wants to learn English, would want English audio and English subtitles, but Japanese as the translation language. That person would maybe benefit from transliteration in the translation language. At least for some of the more rare kanji. I guess Language Reactor is not really aimed for that usecase though.