This is a really good question, and the simple answer is that Slack uses an unmodified version of Electron, and it’s not a good idea to make custom modifications to that project for such a small, niche feature. The cost/benefit just doesn’t make sense for Slack, neither to me, nor to Slack management. But Electron didn’t even exist when Screenhero was acquired, and so the constraints we eventually faced were not possible to predict when we got acquired.
In 2015, when we joined, Slack’s Mac app was using "MacGap" (i.e. the PhoneGap/Cordova model but for Mac). Electron was being used for Windows only, and it was a lot later that it became the platform for Slack’s Mac, Windows and Linux apps.
