Slack's meteoric rise in popularity can be attributed to two major factors: It does it's job exceptionally well, and people enjoy using the app itself.
From a functionality standpoint, Slack's strengths are that it:
- Facilitates communication in the workplace
- Helps teams communicate collaborate across remote distances
- Enables and encourages the sharing of documents, images and other assets across teams
From a design standpoint, Slack is popular because it appeals to all demographics with its visually appealing design, smooth API's and workflows, and it's contemporary and tech-friendly.
Mentioned in this answer
Slack for Mac
A new, improved Slack desktop app for Mac