Creating a web app with a multi-billion dollar valuation indicates it is their strength. Who cares about a little superfluous memory usage compared to that.
Slack didn't win because it was better, it won because businesses thought they were better. That's how most business software works: it creates a problem in the customer's mind in such a way as the customer naturally sees the product as the solution.
Slack is successful because of marketing, not technical superiority.
I worked at a small company that used Google Talk for office chat, and us developers complained because it was difficult to have a group conversation with. We started using Discord (we were familiar with it and liked the dark theme) and it caught on and we were quite productive with it. Our business types saw it and decided to "formalize" it, so they picked Slack and rolled it out company wide. Most people actually preferred Discord, but the business types preferred Slack (probably mostly because of marketing), and now we're all using Slack.
Now, Slack also didn't royally screw up as it scaled up (outages were rare and transparently reported), which is a credit to the developers and PR people. That being said, just because something is successful doesn't mean it's good; it could be, but that's tangential to the topic at hand.
https://en.wikipedia.org/wiki/Glitch_(video_game)
That core team therefore had a lot of frontend web dev experience. It made sense that they'd stick to their strengths