Compared with email, business communication tool Slack lacks one very obvious feature: it doesn't allow users from different organizations to communicate with each other. 

And despite requests from customers, that's not going to change, said Slack CEO Stewart Butterfield. At MIT Technology Review's EmTech Digital Conference in San Francisco Tuesday, he explained why he's not interested in expanding Slack to be a consumer product like Gmail or Skype.

Slack, a chat and collaboration app that archives conversations -- and has been described as the startup looking to kill email -- has raised $160 million and is valued at $2.8 billion. According to Butterfield, 750,000 people use the app each day. 

"I don't have a problem with email in general. Email is a great tool and has the advantage of being the most common denominator and very easily able to cross organizational boundaries," he said.

However, to enable cross-barrier communication in Slack would come at a big loss to productivity, Butterfield believes.  

"When you open your email, it's this incredible amount of context that goes on because it's friends, it's family, it's people you used to work with, people you work with," Butterfield said. "Having all of that in one queue is very difficult to manage."

Which is why those kinds of messages should remain in your inbox and not in Slack.

Butterfield, who co-founded photo sharing site Flickr in 2005, also explained his personal reasons for wanting to keep the app B2B-focused (though it's also used by communities, such as students, for example). When working with businesses, customer support issues are mostly straightforward and revolve around IT fixes. That's not the case in the world of consumer products, Butterfield said.

"When we were running Flickr support cases would be: my ex-boyfriend is a psychopath. I have a restraining order you have to prevent him from contacting me," Butterfield recalled. He said he'd like to avoid being responsible for those kinds of requests this time around.

"Because on the consumer end you get that. You get spam, you get phishing," he said. "You're confronted with the horrible side of humanity. I'm done doing that."