I suspect it was done to resemble motorcycle engines as they were starting to be imported in large volumes on minibikes and that would put domestic builders at a huge disadvantage. I did read an article that mentioned this but I don't remember where it is.
That seems to be the consensus. I was pondering this the other day however and came up with a different possible answer, and that is the use of a torque converter on a larger 10" wheel size, without having to stretch everything out to fit the drive system. In all of those slanted designs, the driver is sitting higher, which allows for more compact placement of the driven and ensuing sprocket/chain/brake.
It seems to me that designing a bike around a slanted engine plate to make a Tecumseh look like a Yamaha is a huge waste of resources, and that instead was the result of building large wheel based "mini cycles," that looked like Japanese machines with integral gear boxes, and not wanting them to look longer, like Tote Gotes with spokes.