Let's see … Here are the other halfbacks, most of them Marshall was better than. Most of them play second fiddle to a five-eighth (Cowboys, Knights, Dogs, Raiders, Sharks) or just didn't play as well as Marshall (Broncos, Titans, Roosters, Warriors).
Cowboys - Michael Morgan/Ray Thompson (Marshall wins) - agree
Broncos - Peter Wallace (Marshall wins) - disagree
Titans - Scott Prince (Marshall wins) - disagree
Knights - Tyrone Roberts (Marshall wins) - disagree
Manly - Daly Cherry-Evans (Marshall loses) - agree
Roosters - Mitchell Pearce (Marshall wins) - agree
Souths - Adam Reynolds (Marshall loses) - agree
Penrith - Luke Walsh (Marshall wins) - disagree
Parramatta - Chris Sandow (Marshall wins) - agree(easy)
Cronulla - Jeff Robson (Marshall wins) - disagree
Dragons - Ben Hornby (Marshall wins) - agree
Bulldogs - Kris Keating (Marshall wins) -disagree
Raiders - Sam Williams (Marshall wins) -disagree
Storm - Cooper Cronk (Marshall loses) - agree
Warriors - Shaun Johnson (Marshall wins) -disagree
So I guess it has a lot to do with the fact, that in general, the quality of the 2012 crop of halfbacks was pretty bad.