Because of the kernel/userspace model of Linux and missing efficient microkernel structure which would lend itself more fittingly to an efficient network tunnel. We're using monolithic kernels because they've attracted the most driver support over the years, but with things like seL4 out in the open, this is bound to change because there's no argument that there's no modern microkernel available to build hardware drivers on. On top of stability, you also get a viable capability system in seL4.