Blunt question I know but this is something I found a bit hilarious and baffling because I'm someone that's fairly adaptable as a gamer and for the most part I'm not that bothered about keybindings these days and being able to rebind them. Some people seem to get really bothered about it though and I was seeing on places like steam some absolute sperg sessions about it.
Do you care about this sort of thing? If the layout is pretty normal with WASD and such I don't understand why people would care that much so I'm trying to go out of my way to understand. One repeating trend I saw weirdly was left handed people complaining about it. My argument is this, there's only so many ways one can rebind the keys when it comes to shortcuts, so I can understand doing it for certain actions, but otherwise it seems silly.
"C" to crouch is helpful in games that have certain contextual movement (sliding/slide jumping and the like.) If I'm sprinting with "shift" and want to initiate a slide it's way easier to use "C" than "ctrl".
Using C to crouch means you have to move your finger off WASD which hinders mobility. I can easily keep shift pressed with the tip of my pinky and use the base of my pinky finger to press CTRL at the same time, and then slap space bar with my thumb to jump while keeping total control over WASD.
That's pretty much mandatory in parkour-heavy games like Warframe where you run, crouch and jump all the time.
It's also more ergonomic and comfortable because my relaxed hand posture is right above those keys. Getting to C requires an awkward movement of either my index finger or thumb.