My favorite comment so far

It is kind of believable, actually. This seems to be the origin of a lot of furry characters out there. I forgot from where I heard it, but I recall once watching a video mentioning Jazz Jackrabbit and how they approached this situation.

Basically, they had no idea of "how to make a rabbit character look hot", and they actually struggled with that concept a lot. They eventually just worked with what they know and gave her more human-looking traits than they did for Jazz himself, such as full clothes and boobs, and indeed, you also see a lot of that is other media like Sonic games. When you really think about it, Rouge just looks like a short human with big boobs if you were to hide her face and wings from view; a lot of nude fanart of her would not even look furry without those 2 things to be able to tell.

I assume X-COM was no different: they wanted to make a race of snake people where all are female-looking, but the designers had no idea of how "female-looking snake humanoids" would look like, so they just added conventionally female-looking human traits onto a snake body in a mermaid-like fashion and called it a day, so it is less "people have a thing for this" and more "the thing was always there, it's just on a snake-looking body now", which eventually turned into a preference for many, and bam, the concept suddenly gets a following.