Why isn't node a dependency for node-mersenne though?

It is. node-mersenne uses node-build-system, which has node (or node-lts, dunno) in its implicit inputs.

Did you mean: 'Why isn't node propagated?'

For the same reasons as why any plugins don't propagate the thing they are plugging in, and why any library doesn't propagate its interpreter:

• There is more than one implementation, how would Guix know which one you'd like?
• Propagation stops you from separately updating the library and the interpreter (with "guix package -u this-package", or time machines, or defining a package variant).  (Actually, "guix package -u this-package" will ‘succeed’ because collision detection is disabled IIUC, but you can't decide which one wins)
• ???

Also, see my answer on 'Is there really a use case for shipping ...’.