According to Twitter, this is namely because of how fast the social network moves, in the sense that people are tweeting out new information all the time, so what could have been at the top of the list a minute ago could have since been pushed to the bottom a minute later. Because of this, display search results that are relevant makes more sense.
Twitter’s senior software engineer Lisa Huang writes, “In order to improve this experience, over the last few months we’ve adjust the top tab of your Twitter search results page to start with relevance-ordered Tweets (rather than time-ordered Tweets). We retrieve Tweet candidates from various sources within a larger time range and rank them with a machine-learned model.”
Since this is machine learning based search results, to improve upon it Twitter is also taking into account things like the number of retweets, likes, and replies to indicate whether or not a tweet is engaging. In any case these changes have already been implemented so check it out if you haven’t done so already.