My guess (and that is all it is) is due to tight tolerances and fitment/rubbing issues.
Although I did not go with the Foose wheel I did go with the size he ran on his car - 18x9 & 19x11 wheels wrapped in 245/35/18 & 325/30/19 tires.
I fought getting the correct offset on the front wheels due to rubbing at full scrub and eventually fabricated a new windshield wiper motor cover under the passenger front wheel due to tolerance issues. I initially thought I may have to roll the front fender farther forward (towards the nose) but ended up not having to after getting the correct offset. The factory roll on the front fender sufficed.
With the rear wheels I had to roll the fenders a bit to get the tires not to rub.
From an online tire size calculator it appears that the difference in Foose's tire size (on his red show car) on the front tires is 9.6" versus 8.9"(his new recommendation). Likewise, the rear tires went from 12.8" wide to 11.2".
By the way, I ended up buying a tool that allowed me measure the offset needed for the actual tire I was planning on running. I mounted the tire on the tool, adjusted the offset, mounted the tool and tire to the car hub, and ran the suspension and/or steering through its paces...adjusting the offset until I found that sweet spot. Then there was no guess work involved.
Just my two cents....or maybe the other tires are easier to get???