linux / windows requires extra testing as well as some adjustments to the software stack (e.g. liquid glass only works on mac); to get the thing out the door ASAP, they release macos first.
Like I notice in Codex in PhpStorm it uses Get-Whatever style PowerShell commands but firstly, I have a perfectly working Git-Bash installed that's like 98% compatible with Linux and Mac. Could it not use that instead of being retrained on Windows-centric commands?
But better yet, probably 95% of the commands it actually needs to run are like cat and ripgrep. Can't you just bundle the top 20 commands, make them OS-agnostic and train on that?
The last tiny bit of the puzzle I would think is the stuff that actually is OS-specific, but I don't know what that would be. Maybe some differences in file systems, sandboxing, networking.
MacOS is unix under the hood so the models can just use bash and cli tools easily instead of dealing with WSL or Powershell.
MacOS has built-in sandboxing at a better level than Windows (afaik the Codex App is delayed for Windows due to sandboxing complexities)
Also the vast majority of devs use MacBooks unless they work for Microsoft or are in a company where the vast majority of employees are locked to Windows for some reason (usually software related).
It takes 15 seconds to verify this isn't even true in webdev, less so everything else.