I'm not involved with the spec discussions, but I suspect it's just because 32-bit floats are heavily preferred on GPUs for performance reasons. If there were a lot of popular demand, I imagine 64-bit doubles could be added, but games overwhelmingly prefer 32-bit; it's mostly the scientific community that wants 64-bit.

Besides, if JS interop were the driving concern, 64-bit doubles would be the default, since that's what JS numbers are.