It was less a intentional choice (early on at least) and more motivated by how painful using aim group chats for software development was. Slack wasn't even centrally managed for the early days of use.

(Although AIM itself started as kind of a unwanted skunkworks project, when there was an off the shelf solution available, people weren't that driven to build their own)

That was the brilliant part of their go-to market strategy. Rather than negotiating with enterprise executives, they let each team start its own microinstance and try it independently. It's much easier to get a 20 person team to try it than a 10Ks of people organization; but word-of-mouth and a quality product can get enough 20 person teams in a 10Ks organization to switch that eventually the whole enterprise signs up.

Or gone with a free alternative like Mattermost & added the necessary bells & whistles.

Mattermost today is extremely competitive with Slack.

If Mattermost is competitive with Slack, that's a serious indictment against Slack.

Mattermost's clients are awful. Just awful. The phone apps are effectively unusable (incredibly buggy and suck down battery like no tomorrow), and the desktop electron app is a complete hog. It doesn't even handle its faux tabs properly (takes half a second to switch, and it resets the position to the end every time you change a tab, which is... dumb), and the scrollback handles high-volume channels very poorly (I have to click "Load more messages" at least 20x times every time I want to scroll back to the beginning of the day). Now imagine every time you switch tabs you have to click "Load more messages" a bunch of times to see earlier messages because it resets when you switched tabs.

If this is the future of messaging, then holy shit. It's almost a parody.

Mattermost is a pale, pale shadow of Slack. It's nowhere close to competitive and I doubt it ever will be.

Mattermost idealistically might be competitive with Slack, but right now it isn't. Most people who only care about usability and features will choose Slack over Mattermost. Just use the apps for a couple days and the differences and issues between them are clear as day.

I'd like Mattermost to succeed. But it needs to do a lot of catching up still.

Before Slack there were other examples in corporate chat solutions like Lync which had a good run. It's not like corporate messaging is new. Wonder why that was missed. AOL too consumer focused?

They more or less stopped development on aim like 5-6 years ago.