My personal reason was observing the huge amount of investments into Javascript and its infrastructure (by Google and others). After Node.js was released it was a rational bet to make that Javascript would dominate, both server and client side. As someone who often does both front end and back end, it felt good to move to Javascript. It's got to be said that keeping a "common" directory around with shared code for both front- and backend is still a PITA, but still better than most other alternatives.