RG3's jersey was third in sales from April 1 to May 28 (behind Manning and Tebow), but rookie jerseys didn't go on sale until April 26. He's the leader here in May.
Maybe it's the fact that RG3 prompted the first instance of "RNOB" (aka "Roman numeral on back," per ESPN.com). It probably has more to do with Washington's long-suffering fan base, which hasn't been entertained by a successful franchise quarterback in decades. (Flashback: 'Skins fans went so far as to make Donovan McNabb's jersey the No. 2 seller during parts of 2010.)
If you're going to fork over greenbacks for an official jersey, you want to know the name on the back is in the team's long-term plans. Thousands of residents within 100 miles of the White House are waxing cars with burgundy rags emblazoned with the names of Heath Shuler, Gus Frerotte and Jason Campbell. The hope in Washington is that this quarterback's name represents something different.