But they have to support multiple mail clients anyway: macOS Mail is based on Webkit, Thunderbird on Gecko (Firefox), I guess it's Blink for many Android clients, and then there's a hundred versions of Outlook.
If you spend all your time making the output work cross-platform, I could definitely see not wanting to go on to spend a lot of time working on making the editor cross-platform. It's the same kind of tedium, but without much possibility for code-sharing.