You know, I am now mildly concerned that there are certain people, once you get them to accept that “cis” isn’t a slur, will start arguing that certain slurs aren’t actually slurs, using whatever misguided/wrongheaded logic they used to convince themselves of cis = not-slur. That’s going to cause some headaches.
For anyone who wants to know the real reason, “cis” (short for cisgender) was originally what is known as an “outgroup label”. Such labels are used by members of a social/cultural/religious group to refer to people who are not part of the group, because saying “non-[us]” or similar negative constructions are sometimes awkward and/or self-othering. For a few examples, Jews refer to non-Jews as Gentiles, nudists refer to non-nudist as textiles, and autistic people refer to non-autistics as allistic. Sometimes outgroup labels can be problematic, and some people find being personally referred to by an outgroup label to be disconcerting because it’s not a self-identified label, but the general idea of it is not a problem in of itself.
What makes “cis” different from most outgroup labels is that some non-trans people are taking it up as a personal identity label. And some other people find this to be deeply weird, because to them labels are about how one deviates from the “norm”. So it doesn’t make sense to them to specifically identify as a trait which is considered a societal default. (That being said, most people who are straight/hetero *do* identify as such, they just rarely think about it, so even people who think that way usually make an exception already.)
“Shemale” originated as, and is still largely used as, a porn term. Even ignoring the term’s inappropriate implications, and that it has been used by hate groups in the past as a genuine slur, there’s still the matter that most trans people would prefer not to be associated with porn.