The heck? Even though it's integrated directly into the OS?

I'm hoping that the explanation is along the lines of Apple building their own Twitter client to replace the existing one.

Twitter having its own desktop client made it exciting like a whole new internet protocol: Usenet, the Web, IRC, email, Twitter. Having it just residing on that terrible website demeans it.

Having said that, the iPad client is vastly superior to the desktop one anyway. Although the select-text-and-tweet-it feature was a workflow with a unusually profound impact on social media.