When people used the @-symbol to mention other users and the hashtag to refer to topics, twitter build features to support these conventions—linking @-mentions to a profile and hashtags to a search page, notifications, replies ...

When people used "RT" and copy-and-paste to retweet people, twitter build the retweet feature.

When people linked to images, twitter started showing images directly in the app, and later even made it possible to upload images.

When people posted screenshots of tweets, twitter should have asked why and how people were doing that—mostly to express outrage, often to sic their followers on the OP, but also just to "steal" tweets, so that they, rather then the OP, get the retweets. And learning that, they should have stopped supporting images. Instead, they build the "quote RT" to make that easier.