disowning Honest Abe?

Just saw Romney give a speech where his trinity of great Republicans was Teddy Roosevelt, Ronald Reagan and Bush I. The PBS pundits noticed that he skipped W (which makes since given his low popularity even among Republicans). But I scratched my head as to why those three beat out the guy who freed the slaves and kept the country together... I mean, it annoys me when the Republicans do the "we're the Party of Lincoln" thing ad nauseam, but it seems weird to have Romney not do it.