Because it can be used as an exclusionary term to minimize cis voices. Stay with me, this isn't going where you think.

When trans folks engage in community discussions, it's very typical for cis people to insert themselves into the conversation so they can tell trans people who they are and aren't, what they should and shouldn't do. This actually happens with a lot of minority groups hoping to have serious discussions in public. Black folks hear about all lives matter or black-on-black violence, atheists here from religious proselytizers, etc.

So when trans folks - very rightly - let cis people know not to talk over them in their own communities, bigots believe -very wrongly- that their rights are being abused. Therefore the conclusion that "cis" is "discriminatory." See also "anti-white racism" "Men's Rights," etc.

All of these grievance perspectives are based on real-world difficulties, but provided without context. There are certain specific situations where it may be disadvantageous to be male, white, and cis. But those specific circumstances are not a part of systemic bias. If you don't care about context, and you don't care about systemic bias (particularly because it tends to benefit you), it's easy to view these isolated situations as a cause for victimhood.