You can get a much better description of the differences between units.

I believe the reason is the primary connection is not just to a PC, but to radar, chart plotters, etc. That's why they are generally connected by NEMA 183 (or newer) devices which have RS 232 interfaces for the most part.

http://www.panbo.com/cgi-bin/admin/mt-search.cgi?IncludeBlogs=1&search=AIS+and+RS+232