All those articles try to paint it as "open source vs closed source" or "simple vs complex" but the answer is pretty simple: the IRC itself was missing many important features. No offline messages, no multi-device support, no threads, no message archives, no message search, no inline images, no mobile phone support.
Sure, you could have added this as a separate component.. but as long as you claim "we are using IRC" there would be that one person without bouncer and web browser who keeps asking the same question over and over and whines if someones posts an image.
So Slack won on features. Turns out people can tolerate slowness and crowded interface, as long as it gets stuff done. Who would have thought.
