why a Ford 9"?

I'm going with the apples and oranges here. And yes, I run both 8.75 and 8.25 rears in my cars.
BUT.....
Talking hot rods, modifieds, customs, drag cars, you are going to find more 9 inchers than anything. Put your micrometers down, close your gospel magazines and forget about your bearing load torsional stress charts. The fact of the matter is that to the rest of the world, the 9" is perceived as the best option.
Think about it. Back in the day, you could go to the boneyard and rape pretty much any Ford for it's beefy rear. You couldn't do that with any Mopar for it's Dana 60.
Now, you'll scream that the 8.75 is just as strong. Which might be true, but, it has never been billed as such, even from our dear Mother Mopar.
If you were building a high performance car, you wanted a high performance rear end.
Ford 9"
Chevy 12 Bolt
DANA 60

Those were the diffs of legend.
The Dana was the best but too few and too expensive
The 12 bolt? C'mon, really?
The 9" was/is abundant and cheap.

Economics, advertising and perception. If you look in the aftermarket today, you'll see virtually every street rod/custom chassis uses the Ford rear. It's just the way it is.