Why is "Multiplexed, non-blocking I/O, [..] much more scalable than thread-oriented, blocking I/O"?

Question

I'm reading about Channels in the JDK 7 docs (here), and stumbled upon this:

Multiplexed, non-blocking I/O, which is much more scalable than thread-oriented, blocking I/O, [...]

Is there a simple explanation as to why this is so?

Answer 1

Because a thread stack is usually much larger than the data structure needed to support an async I/O connection. Also, scheduling thousands of threads is inefficient.