For the past few years OnePlus has made good smartphones. The OnePlus One, Two, and X have all been popular phones. But until now you had to sign up for an invite and wait a long time sometimes to get your phone. In the days of Amazon Prime 2 day shipping it is hard to stomach a 2 month wait for a smartphone.
The OnePlus 3 will launch on June 14th without needing an invite. Whether this means they have enough stock to not sell out quickly or not is still to be seen. It would be a waste of time to make a big display about not having invites, then having the phone sell out and have a 2 month wait.