When you are dealing with muni contracts and preferred vendors and such, and basically getting the vehicle that fits the spec, this is what happens. A pattern begins- maybe 10-20 years ago, and it just grows from there. Hey the 7. 3 Econoline wasn't a bad bus as we know, and that's the bus that got the stamp of approval. But, the bean counters don't care about a troublesome engine as much as getting the bus into the fleet and satisfying the contract. I have personally lived through a chassis changeover on the fleet I work on, and it was a huge learning curve for the new manufacturer. Now that they are in, they're untouchable, even though the product has gone downhill.
Add to the fact that Ford does make the most versatile line. The possible chassis/ cab combinations are dizzying.