Look at the GAF thread for Horizon and compare it to Zelda's thread. Zelda even has like 3 active threads going on and the main thread is still huge.
I'm also wondering why people aren't into it. It's one of the games I'm most interested in playing that's been shown this week. Looks fantastic!
It's a new IP so the masses don't know about it like they would a Zelda. Sony also hasn't really hyped it up all that much. It takes time for a new IP to gain traction.
It's still really far off and I think that's the root of it. People are getting burned by delays so much that they refuse to get excited about things so far out.