I can understand why small teams with a web background might pick Electron instead of going native but I don't understand why the larger companies like Slack don't write a native app when they have the resources to hire the talent.
As a dev I'm biased of course and this might not matter to most people. One thing though, you don't have to write the full app three times. For example all the business logic can be in C++ and write a native UI on top of it.