While it's important to know what tweets a user is interacting with, the personalization option can surface more things as "You might like", "You follow x, and he liked", "Interesting tweet on the topics you follow/might like" and so on. Whether or not you choose to interact with anyone or more of those is a signal, and helps with advertising (if advertising is a question of "What tweet/content you are likely to interact with?"). Other than brands, most twitter influencers, or would-be influencers end up spending money on twitter ads to gain followers.
With a reverse chronological timeline, it's hard to surface those tweets from outside the network without a jarring experience. They also realize the power of defaults and hence they decided to take away the default option.