Perhaps it’s as simple as photos being more appealing to a broader audience than tweets. But I say part of Instagram’s success is that their interface is simpler, and the rules for what you see in your feed are like what Twitter’s used to be: a simple chronological list of posts from the people you choose to follow. Insert your own “Correlation is not causation” disclaimer here, but it seems to me that Twitter’s slowing growth corresponds pretty closely to its complexity increasing over the past few years.
I remember Twitter being easy to explain, even if it wasn’t conceptually easy to understand: “constant 140 character snippets of whatever from people you find interesting”. Twitter is no longer that simple; its complexity has made it more difficult to explain and understand.