Twitter, as it has evolved (or been driven depending how much you trust the vision of its management) is not a lucrative platform. It needs either to change how it connects payers, payees, and audiences drastically (and not just be pitched differently), or it needs the industry to change underneath it. The latter option is not actually that unlikely at the moment.
Every ad I see makes Twitter maybe 5 cents? I'm guessing they earn $40 / year on me (I'm not a heavy uesr). I wish I could just pay them $40 and not have any ads inserted into my stream for the rest of the year.
Facebook's per-user revenue in the US as of Q4 2015 was around $11/yr (source: http://www.businessinsider.com/facebook-average-revenue-per-...) although its grown by 50% yoy.
Chances are if FB / Twitter / Instagram had high confidence that could monetize their product with a subscription model averaging even $10/user a year I think they wouldn't hesitate to implement such a model.
I think comparisons with Facebook are a big part of Twitter's problem. They aren't Facebook, never will be, and shouldn't try to be. If they use Facebook numbers as a benchmark, then they are totally screwed.