A few reasons I could see 1.) Instagram brand is much stronger than Facebooks 2.) Instagram follows some similar design and user experience patterns to twitter

A few reasons I could see 1.) Instagram brand is much stronger than Facebooks 2.) Instagram follows some similar design and user experience patterns to twitter, i.e. username based account, people using stories a lot of times as mostly a text based posting platform, etc 3.) Celebrities and newspeople already use instagram for updates, so the transition would be natural 4.) Twitter has never been in a weaker position with the site being limited