In reply to Frankie Jarrett.

This is a footnote located at the bottom of their release post that explains why it wasn't added earlier.

Q: Why are you releasing Two Factor Authentication now? Why not earlier?

Two Factor Authentication has been in development for the last few months. It is a complicated change which requires additional support resources, administrative capabilities, changes to all applications, mobile and desktop, and extensive testing. We were about a week from release, with just a few small UI tweaks to simplify and clarify the usage experience.

We have decided to release it immediately, despite the remaining bits of clunky-ness: the feature works and it does provide a significant new level of protection against unauthorized access to your Slack account. We will be improving this feature in future releases but the feature functionality is what is most important right now.