Because every language and tool is part of a wider software ecosystem, and it's reasonable to ask how any given language compares to the other options that you have.
Usually I pick languages based on the ecosystem and not the other way around.
One thing that Grain has going for itself is the toolchain, just like AssemblyScript.
Most languages targeting WebAssembly, instead of directly generating wasm files, use a klundge of Python, JS and Java tools, with Emscripten being the most notorious in that regard.
