Maybe this? But who really knows, with Valve:

"SteamOS will be available soon as a free download for users and as a freely licensable operating system for manufacturers."

Same reason Android was announced before the first phone: because surfing the wave of rapidly improving hardware is part of the plan.

Casual vs Hardcore gaming. If they announce the console first, and it's not able to run the latest and greatest games on their highest resolution they stymie the idea of being able to run SteamOS on a high end gaming rig.

This way people are thinking about how they can run SteamOS on their existing hardware before thinking they need to run it on a Steambox.

My thought is to temper disappointment. If they announced the steam box today and it ran a flavor of Linux it would be a disappointment. If we know about steam OS for 48 hours before the steam box announcement, we would all expect the steam box to run steam OS.

Valve likes to escalate announcements. It would be very "Valve" to announce the OS, which can run anywhere, and then next announce the box they want you to run it on.

well, the next icon on this page http://store.steampowered.com/livingroom/

sure looks like a box enclosing the circle that turned out to be the OS. It's not really a separate announcement as this is all part of one unified event.