Server side web technologies/languages change/shift in popularity really fast. As a freelancer, the best thing you can do is learn how to learn things fast. Most of the patterns translate over between languages and technologies though.
Client side work is even more wild. I remember when everything was just jQuery spaghetti. Then things moved to KO/Ember, then to Angular in around 2012, and now everyone I know seems to be moving towards React. A lot people were disenfranchised by the Angular 2.0 announcements over the past month.