It wasn't a technical decision. Apple decided to veto the use of any Khronos IP such as SPIR-V or GLSL due to a private legal dispute which was never elaborated on. That left the committee with no choice but to reinvent the wheel, and now we're stuck with WGSL forever.

It's not the end of the world for web-only projects which can just target WGSL exclusively, but it's a pain in the ass for cross platform engines which now need to support Yet Another Shader Backend. From the old minutes:

> Eric B (Adobe): Creating a new high level language is a cardinal sin. Don’t. Do. That. Don’t want to rewrite all my shaders AGAIN.

> Jesse B (Unity): If we can transcode to HLSL to whatever you need, great. If we can’t, we may not support your platform at all.

> Eric B: Would really not like even to write another transcoder. If there’s an existing tool to get to an intermediate representation, that’s good. Would suggest SPIRV is an EXCELLENT existing intermediate representation.