Ecosystem lock-in. I remember shortly after Twitter launched there were many copycat microblogging services and the community had grand ideas of an "SMTP for Microblogging" so Mentions, Retweets and the like would work in a distributed fashion between service-providers, you could even run your own fully federated microblog off a Raspberry Pi in your garage if you wanted.
Twitter kept themselves in the lead - not just by delivering a superior user-experience (Nazis and trolls notwithstanding...) but also cultivating a developer community in a way that kept people off rival platforms - before stabbing them in the back in recent years (e.g. no support for polls in third-party clients).
This announcement is a continuation of the strategy: Twitter is making more data about their platform available, other platforms aren't as open, so people will stick with Twitter because their data is available - and it also benefits them directly because these "Premium" APIs do cost money to access:
> Pricing for these elevated tiers of the Search Tweets API starts at $149/month
Their search product started as a third-party integration (Summize). Their official iOS app started as a third-party app (Tweetie). They've benefited immensely.
