> Certainly having a larger stdlib helps Go projects keep their deps down, but I don't think it's the real reason

I disagree: having a big stdlib that encompasses a huge chunk of common functionality (logging, a plethora of network server/client protocols, etc) means that for basic needs, the question of 3rd party libs never arise, whereas for other language, the question is not if you need a dep, but which one. Making gorilla easier to find won't undo the cultural reluctance of adding a 3rd party dep when using the (very pragmatic, IMO) stdlib server is adequate to the task.