Because the 80’s was over 40
years ago and thankfully language evolves over time. Cis gender was coined by cis gender scholars in the 1800’s and has nothing to do with sexuality. It’s not new and it did not come from the trans community. Also, the term bisexual tends to confine people to being attracted to cis gender masculine or feminine presenting people which leaves out a whole lot of other people. Pans people want the world to know that they are attracted to PEOPLE no gender or sexuality specific expression required.