From my perspective, it went from nothing to huge success overnight.
Then within a month every other team I talked to was also using it... blew HipChat right out of the water.
I would attribute it to the amazing UX and well thought out feature-set. Integrations and per-channel notifications were both essential features and easy to find and figure out how to use.
I also wouldn't discount the design, their choice of color palette and styled stripes with the # managed to set them apart while still looking professional.
I don't know what contributed the most to my adoption personally, but whatever it is, they've got it.
For the general functionality, they're the same thing. It's IRC rooms and direct messages with persistent state across devices and support for pasting images and files.
HipChat does have a native Windows client, but it's months behind the OS X version, doesn't work with touchscreens and has known issues that take months to get fixed. I generally end up using a Chrome wrapper on their web client, which is how Slack does multiplatform anyway. Not a great solution (Chrome has a tendency to pop the software keyboard up all the time, even when you have one plugged in), but it works. If you're an OS X shop, the native clients may be a plus for HipChat.
As far as functionality goes, one feature that (AFAIK) is missing from HipChat is the ability to log in to multiple organizations. If you use it at work and at home, you'll be logging out and in on every device every time you want to change modes. One of my side project HipChatters uses it for his day job and has this issue. Slack's client lets you sign in to multiple teams (haven't tried it, but I see it in the menu).
I've spent much more time in HipChat, so I've experienced a lot more random issues there. Stuff like several hours of chat history being skipped when I scroll up (fixed by relaunch if you notice it happened), constant failures to reconnect after waking from sleep (again, fixed by relaunching), phone notifications popping up for every message even though you're currently in the chatroom on your phone having a conversation with someone, etc. It's possible that Slack has similar bugginess that I would have hit if I used it as much as I use HipChat, but I don't have enough time in it to say.
That's the TL;DR; - here is a story which may give you no more information than that:
I was working out of AlleyNYC when HipChat was getting big and they started using it internally and told all their companies that they needed to get on HipChat so they could get door/package notifications, announcements and talk to the staff... "by the way, it's awesome!".
We installed it, setup our accounts and the whole nine yards, but can't remember any point where I found HipChat showing me any value over Google Hangouts.
When one of my colleagues mentioned Slack (weeks later) it was very casual and when I signed up I found lots of interesting things readily available to me that I hadn't found other places (integrations, style, notifications, slick mobile apps, etc.).
Now, that's my subjective experience... was I in a bad mood that day I was told I needed to use HipChat and din't like being told what to do? What I in a particular good and open mood when Slack happened to be mentioned? If this had happened the other way around would I be a HipChat user?
I have no idea... my hunch is that the Slack style interested me more and that their startup tutorial presented their value more plainly and obviously than HipChat - but it could be as simple as the different context that I was exposed to them in.
Support for multiple organizations.
I think document support is better, but my HipChat workflow doesn't use many docs, so I could be wrong.
All in all, I think the Slack team studied core user flows and made polished solutions, whereas HipChat sort of does but is painful. An example would be reading scrollback or searching in HipChat. It is dead slow and clunky.
Dunno if there is any correlation with that. But in general it works as expected.
It seems well suited to purpose, and works fine for me. The only gripes I have are minor (for instance, their iOS offering has this infuriating behavior that it will notify you for direct messages, and throw up the old red dot on the app icon, but then provide no ability to see where the message came from when you open the app.)
In light of how well it works, I am mystified as to how slack has managed to come in to the exact same space and carve out such a massive valuation. If there was ever an industry ripe for disruption, the "modern chat client with goofy icons that allows you to post gifs and search your chats without arcane IRC syntax" did not seem like one to me.
Slack is a substantially better experience than Campfire
https://slack.com/integrations
All snark aside, the lion's share of the integrations seem geared toward technology workers.
I am pretty sure even founders don't know. It just happens with a combination of lot of luck and a very good product.