Because the devil is in the details. I will explain.
Because Kevin Systrom and Mike Krieger by 2010 (release date), or rather, much earlier analyzed the successes and failures in sharing content with other social networks. And most likely this analysis allowed when creating Instagram to take into account all the factors that affect the popularity of social networks.
At a minimum, it was necessary to analyze the CA of social networks and what is popular. As some Connoisseurs have already mentioned here, pictures are popular, because most people are visuals. I do not presume to affirm who the majority is, because I have no data. But whoever has that flag in his hands.
Personally, I am for this approach - analysis, data collection +/-, compilation, product, A / V testing, we leave what works best (approximate scheme).
IT is not a collective farm entrepreneurial business, which appears only from a conversation between two friends in a bar under the libations of Falern and someone's idea to earn money, because the idea seemed to him successful. As a result, most entrepreneurs burn out.