Because it's easy for everyone to use, even non-technical people. And it requires no maintenance, so the people running the project aren't spending time doing things that are not running the project.
I agree, it's just sad to see several areas of the market move towards centralized closed source solutions. It's as though a new generation of developers don't even know what open source is, including the dangers these "easier" solutions bring to the table and how they shape the future.
So why isn't the open source community moving towards an open source, all in one solution like this? To just ignore the popularity of Slack, and insist that everyone just use IRC and handle the other stuff themselves is just foolish.