Node is built for massive scalability on applications that (mostly) pass text from one source to another. Thus, having to convert the encoding of every string that passes through node can be a bottlenck.
Felixge has a good writeup of this: http://debuggable.com/posts/streaming-utf-8-with-node-js:4bf...
It should be noted that "most" here presumably means "most characters in an average English or western/central European language text" as out of the ~2^21 (~2 million) Unicode code points, only 128 are represented using 8 bits in UTF-8.
ASCII is not an option for languages other than average English with poor typography and inability to deal with foreign names and addresses (e.g. LinkedIn made horrible mistake of using Latin1 initially. I still have contacts with &xxxx; visible in their names).
I think node.js should use UTF-8 by default, and require users to consciously switch bottleneck parts of their apps to ASCII.
But yes, I agree that UTF-8 would be a better default than ASCII unless someone provides hard evidence that encoding/decoding is a severe performance bottleneck in most real applications. (even then, I'd default to the "correct", not the fastest)