There are probably multiple reasons for that. I'm not aware of any GM wheat grown anywhere in the world...or there's been research on it. I think it has been the amount and the focus of the research that's been done in terms of traditional breeding, breeding programs, and investment in wheat. Public sector investment has not been focused on wheat. That's an area where you now see a lot of marshalling of effort and resources starting to happen in the public sector.
On March 3rd, 2011. See this statement in context.