JS is mostly 'unsuitable' because of the lack of integration into GPGPU.
This is one of the reasons I was hoping WebCL[1] (now dead[2]) would take off. It would have enabled GPGPU work that is pretty much a prerequisite for anything that would be taken seriously by the community.
There was talk of ARB_compute_shader support as mentioned in the second ref link, but I haven't seen anything else in that area.