Image and video posts get way more activity than text posts constantly. People don't just want text posts and no one wants to be reddit's free image host like imgur used to be.
Who cares what gets most activity? Arguably the text/conversation content on Reddit is massively more valuable than the images shown there. (Not in the sense of market value, in the sense of value.)
Reddit cares as a business, more activity means more chances to make money whatever scheme they choose to use. Users also want more activity because that gives them more to do on the site.
