Because right now you have to intentionally "hack" your server to include it, once it's part of some standard components, it will be used automatically, or at least will be easy enough to add. I think nginx is working on including it in the default setup.
To sum it up, clients (SPDY enabled by default in FF13 and already in Chrome) and servers (Apache, nginx, Jetty, node, Google sites) are becoming available, some niches (mobile, high latency) are bound to benefit a lot and it's a good (better?) solution for the general case.