The main reasons:
Over a websocket it’s faster.
Rendering on the server to do this was a really inefficient idea in almost any other language but the shocking speed of Phoenix rendering makes it plausible.
People are really tired of Frontend JS framework Hell when it’s not necessary.
Handling how to deal with each structure of data that can be sent to the client is how it grows big enough that people start asking for frameworks. Not doing that avoids it entirely.
Basically, this makes sticking to server side in a world of overzealous JS entirely feasible with virtually no negative trade off (and a lot of positive ones). Aside from the case where you actually need a fully in browser application…this could bring balance to the universe. 
