Believe it or not, I think it's being held back for the 4K screen.
I think the Mini's about to go mobile.
Mobile is where everything's at now, and 'Home on iPod' never really took off.
Users can take their computer from home to work without a hassle, in their pocket.
However, in true Apple style, they want to sell you two (!) screens for the privilege, and so the product cannot debut before the next display upgrade is ready to ship.