Yup, lack of aerodynamics. Great lift though.
I was reading in either Plane & Pilot, or Flying magazine about a guy from TX that bought an AN-2 while on a business trip to the former Soviet Union and brought it back to have it restored. It's cruise is maybe 130knts. They have a procedure in the operating handbook that is to bring the plane to 34knts IAS and land it pretty much anywhere in case you get lost in fog and can't find the airport. I guess they didn't come with an IFR package.