Snake IS loved. And for good reasons, and he get massive points for being the first third party revealed for Smash. His moveset is cool, he brings the music of his game and a part of his world in this one and it's great to have him interact with Nintendo's.
But if he wasn't in, would you really think "Something is not right here"? It would be cool to imagine for sure, but would it really go further than how some might feel for Dante or Master Chief?
In contrast, you have the DK franchise, one of the most classic Nintendo IP. Is it too much to ask for another character for it? I wouldn't be mad if Dixie got in, ideally her and the King would be in. But in my opinion, the latter has more potential for interesting stuff with him. Movesets, cutscenes if there's any, more personnality in general and the satisfaction of playing as someone you never can. You are right saying K.Rool is pretty much irrelevant nowadays, but he is still by far the most memorable villain of the franchise. As good as Retro's games have been, does anyone care about Lord Fredrik? I'm not saying there should be a mandatory villain for every franchise, but in DK's case it's so easy that it can't help but feel like a glaring omission