The folks at Yamaha aren't fools when it comes to marketing. My guess is that if there was a big enough market for 76-key hammer-action keyboards to justify the manufacturing setup costs, they'd sell them. Very few makers offer a 76- or 73-key board with hammer actions; the only one I can think of currently is Nord. They use actions off-the-shelf from Fatar, so it's easier for them to whip up an enclosure that houses an already existing action. Plus they come at boutique shop prices. The CP4 action is designed ground-up by Yamaha and particular to the CP4 and CP40 line (unless there are now newer products using this most excellent action; I may be out of date.) And while it's not an inexpensive stage piano, it's definitely an incredible value IMHO.
I thought I wanted a 76-key digital piano too, when I finally got around to replacing my beloved but aged Ensoniq MR76. What I found is that modern keyboards like the CP4 waste far less space, and the CP4 fits in the same case as my MR76! That's the beauty of placing the mod/pitch wheels above the keyboard rather than to the side. So now I get 88 keys in the same space I used to use for 76, but can play boogie-woogie in C with deep bass while plinking the top octave, something I could never do on the MR76. I also love that I don't have to hit an octave-switch key to use low C or B (and then have lots of notes below that I never use.)
Still, 6 inches is 6 inches, and if they did make them, I know people who would buy them. But probably not enough people. The difference between 52" and 46" just isn't enough for most people to sacrifice the full 88, especially with a great piano like the CP4.
I've always wanted to see someone make a 77-key board from low C to high E, which would shave that same 6" and I'd never need the octave shift. But oddly I don't get agreement on that even from other keyboard players, so it'll never happen. There are others who'd like to see one starting from low A, just like an 88, but it's a minority. Most prefer the classic E to G, which I assume is what you're asking for.