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.
1. It delights me by doing what I want.
2. It makes normally complex programming tasks easy (try metaprogramming in php).
3. It is simpler than I originally imagined.
1 & 2 generally means... it's a "hackable" language (ruby is another good example). 3 is icing.