Because it pretty much only makes sense for Samsung TVs and smart appliances since it scores 3% on the benchmarks vs V8.

It's too big for most embedded devices, too slow for general computing, and if you can run something 25% the size of V8, you can probably just run V8. If for some reason that size and speed profile does fit your niche and you aren't Samsung wanting to use their own software, then Facebook's Hermes looks better in terms of licensing, speed and binary size and writing compatible JS for it isn't that hard.