[Woodz428]

Primarily it is port design, although the difference also requires a matching intake manifold. There is a lot of discussion regarding which is best, but my vote goes to the medium riser in most applications.
I believe Dove and/or Blue Thunder may make some intakes, the reason most parts you see are for medium riser is just sheer numbers. They were made in much larger quantities than the others and replacement parts were also used on other engines so they were available through the parts department.