The problem with the chronological feed is that you get people's posts bunched together. If a friend is having a night out and posting lots of selfies, meal pics, drinks pics, etc then that's all you'll see when you log in. It'd just be one person's updates for pages on end. That's a bad provably experience that stops people coming back to the app. The algorithm tries to improve that.

Also, and I don't know about Instagram, but on Facebook if you have 250 friends you could be seeing as many as 1000 posts a day from your network. You'd never see anything more than an hour old if it was chronological - imagine all the stuff you'd miss any time you didn't choose to scroll back.

Chronological ordering is awful.