Non-trans is still othering to trans people in its own way (there's trans people, and then there's everyone else). It's also kind of imprecise and awkward. To use the analogy from the comments above, it would be weird to describe someone as "non-black" or "non-blonde-haired person" etc.

Cis is a term that allows for better discussion and examination of intersectional issues. You could, for instance, compare the experiences of black women who are cis and gay, and black women who are trans and gay and figure how those two specific groups might face different unique systemic challenges. Once you tease apart novel issues at that level, you can better address them.

The right always is trying to muddy that puddle because they hate the clarity it provides, and they percieve the examination of the issues as a threat to the heirarchies from which they benefit. Same reason they have tried to turn CRT into some kind of toxic term, or why they came up with "All Lives Matter".