Short answer - because the original events were called “Pride” and other events that followed that model and style can literally trace the name to two organizers of the original event, Brenda Howard and Robert A. Martin.

Long answer…

What is important to remember about Pride is it is specific. Gay, Lesbian and Bisexual liberation marches pre-date Prides but they were more like a conventional protest and they were poorly attended because you had to expect police violence. They were dour, focused primarily on the pain and hardship of oppression. It was mostly people dressed to look respectable marching with signs to appeal to the cis/hetro masses in a “hey look we’re actually just like you!” kind of way.

“Pride” was different. They organized the first event around the concept of Independence day style activities. It was supposed to have the feel of an emancipation celebration and was originally intended to become a National day of observance of the five days of Riots at Stonewall, something that a lot of queer people decided to rally around as essentially the literal fight for independence of queer culture in the US. Shortly thereafter a lot of cultural aspects of Queer community done for fun that actually create a culture like Ballroom culture, Drag performance, dance, theater, caberet, burlesque, various bizzare kink related specialities were spotlighted. Pride took all that stuff that was happening in the shadows and turned it into a public festival. In part it was intended as a “fuck you we are not afraid and there is more of us than you think” but it also gave the public a look at the spectacle of open queer joy. That it was fun and weird meant it became a proper festival. It spread and other events that followed that format also became “Prides”. Over time other communities and sub groups within the growing coalition came to define their own means to celebrate together and also came to call then things like “Trans Pride”.

So at least in part the “Pride” portion is a historical naming convention for a very specific style of event and festival with a tracable history. It is helpful to understand that “Pride” has a secondary and silent implication of Pride Event "Woo Happy Pride! " is at some point like wishing someone Happy Christmas. “Proud” is in part an event theme that euphemizes that original “fuck you, our culture is valid and we won’t be shamed out of the public eye.”

Someone going on about “cis pride” is at some point basically just trying to carbon copy a format of protest made for a specific purpose while entirely misunderstanding the original usage. Some argue they don’t really need a specific public culture festival or a protest because they are the dominant culture. They get their culture fest from national and religious tinted celebrations and they are accepted as a norm so the protest element is unnecessary. It more comes across more as someone who just doesn’t like how queer people have claimed a slice of public space and want to have yet another party to celebrate themselves. It’s like throwing an Independence day style celebration but when there is no commemorative event at it’s core and no independence that needed to be fought for at all.