You say "just a snake and some platforms," but that snake is actually an incredibly complicated set of physics calculations. It's not like, say, Mario who is essentially just a set of animations laid upon a 3D model, the entire thing has to be handled by the CPU on-the-fly along with taking into account friction, weight, etc. This
Digital Foundry video explains it in a bit more depth.
I'm not implying that I know for certain what the Switch is capable of, but Snake Pass isn't as undemanding as many think it is; it's why even the PS4 Pro couldn't handle it at 60fps.