If we play the guessing game I'd assume they'd like to make more money, and doing what it's been suggested on this thread is most likely not going to provide any benefit for Slack but more like the opposite.

To make my point clear, I don't believe the current app is so terrible (performance-wise) that's making Slack lose any significant amount of users/revenue.

On the other hand, adding new features to the (cross-platform) app, fixing bugs, etc in a timely fashion will help the business. I'd also include polishing here but rewriting the whole application from scratch for different OS is, IMHO, not worth it.

They have all of that money because they have made good decisions about their product so far. They focus on what matters most to users: features and reliability, not the framework the UI was written in.