Jesper is correct. There's too much out there to be an expert in it all. The old "drinking from a firehose" analogy comes to mind.
The dilemma arises, however, in deciding which to deeply delve into. One must learn enough about the various choices to make an informed decision on which to learn in depth. And that's a juggling act between sometimes-conflicting criteria. Which will make you more productive? Which is better suited to a the project? (And this could change project to project). Which has better community support? Best documentation? Which is (or will be) in demand by employers?
When it was hardly known by anybody, I made a guess that jQuery would be a rock star among the JavaScript frameworks. It was more than a lucky guess, but there were a lot of contenders at the time. (Does anybody even remember Rico?)
A perfect current example is JavaScript MVC frameworks. Choose Ember.js? Backbone?
AngularJS? Other lesser-knowns?