The high velocity web tech landscape and a startup (velocity) mentality are more likely the reasons why this was not made originally (most likely a good trade off, in particular the startup velocity argument). I'm a little surprised a community like HN don't see this, and rather bash Electron being the devil of all things (as some comments seem to imply). We should rather commend Slack for (obviously) making a decent tech choice (using a perspective where what really matters to them; rapid growth, is/was the first priority above all else, including "technical excellence").