2. lots of devs already know the javascript API very well.. a lot of times these same devs write server backends. why not share code between client and server?
3. (maybe just me) it turns out writing in a language as dynamic and flexible as javascript is pretty fun
The way Ryan explained it is that moving past that performance barrier is a solved by a single threaded application that can respond to event based input output. He liked Javascript because it is such a minimal language which already supports that paradigm and has advanced tools and community.
That's the main point that I recall from one of the talks of his that I watched. There are some tertiary reasons as well which make Javascript a good choice.
Don't get me wrong, I started with PHP and it is solid, but yech I wouldn't call it fun.