Well, trans as a root means across from, or on the other side of.

Cis means on this side of. Both are from latin roots.

When using it in gender discussions, it means someone that isn't trans, aka the gender normative, aka the folks that match in terms of inner and outer gender expression.

Cisgender started out as a term back in the nineties, as a way to be able to refer to the majority that are gender normative with a simpler term when discussing transgender/transexual issues. As you can see, it is incredibly cumbersome to describe the cisgender people of the world without using cis. Pain in the ass when you're writing or talking about the subject. And the nineties are when that kind of discussion became more prevalent.

There's also the fact that people have put unnecessary weight to the word "normal", and tend not to understand the word normative. Because of the way normal has been used for a very long time now, despite it really meaning something that's typical, any use of it implies that everything else is abnormal in a bad way rather than just not typical. Largely because in most fields, abnormal is a bad thing. Abnormal blood work as an example.

So, we have heteronormative and cisnormative for the straights and non trans people behaving in typical ways for those groups as well as cisgender meaning aligning with one's nominative gender.

Now, can cis be used to denote "straight" people? Kinda, but not really. It would be a very unusual usage because straight in terms of non normative sexuality being discussed almost always refers to sexual orientation. Using cis to mean straight isn't unreasonable, particularly since you'll run into situations where gay people and trans people might just use straight as a shorter word for cis-hetero. But you won't see that in anything but casual settings because of the very confusion you're dealing with. Most of my close friends are gay or otherwise under the lgbtq+ heading, and I've never actually heard anyone use cis as a synonym for straight, but I have heard "straights" used as a term that includes cis.

Yay for language!