Quoted from Fortytwo:
I beleive during the tron run they had some USA machines made with one slot. Peoples complained and didn't like the look of it. So they went back to two slot for tradition.
What I heard. Correct me if I'm wrong. I personally like the two slot look.
While that is true, there was also another issue with those coin doors in that some of these did go out on route and there was no cut out for a DBV either. It was a cost-cutting mistake and they learned from it.