The reason behind the change is that the old icon no longer reflects the Instagram community. At least, that is the reason mentioned by Instagram's head of design, Ian Spalter in a blog post. "Our hope is that people will see this app icon as a new creative spark," said Spalter.
I never understand why apps change their entire layout for no apparent reason. If it ain't broke don't fix it! #instagramupdate— Michael Woods (@MichaelWoods) May 11, 2016
The #instagramupdate logo is absolutely annoying, I couldn't find Instagram as I didn't recognise the logo— quisharose (@itsquisharose) May 11, 2016
Instagram's newest UI is clean. Loved it.— Ario Pratomo (@sheggario) May 12, 2016