Slack is protecting their multi-billion dollar business and their investors which are both based in the U.S. If they do not, they could run into trouble and put far more at risk. This does not only affect Iran, it affects other countries too which the U.S. has active sanctions against.
Now, I admit that visiting the countries on the list aren't on my to-do-list, but if for example I decided to make a vacation trip to Cuba I really shouldn't lose my account at any US-based service just because of that.
I hosted my own blog for a short while and got hacked, so I moved back to Wordpress.com.