Casey Newton, reporting for The Verge:
Development of the Mac app was outsourced to a third-party developer, said Jonathan Wight, a former Twitter employee, in a tweet. The Verge confirmed that the app’s development was outsourced with other people familiar with the matter. One of those people said the developer is Black Pixel, a well-regarded digital studio based in Seattle. Black Pixel’s other clients have included ESPN, Starbucks, and the New York Times, according to its website.
Black Pixel has built some great stuff, but this might explain why the new app doesn’t support some of Twitter’s new features.