The amount of people who can authenticate (as in, connect to the app once by linking their account) is limited to a certain amount. Developers are / were given a fixed amount of user tokens.
I keep recommending everyone to abandon the official apps and use e.g. Tweetbot because the official apps suck compared to 3rd party (commercial messages alone is a dealbreaker) - but do peolpe then take my advice and go download tweetbot only to find they cant auth? That would be bad...