The only reason I can think of, and Im saying this without any research on Humminbirds products, is because the smaller screen typically has better resolution.
From my person view point, structure fishing really doesnt require a larger unit up front. A 998 would work perfectly considering most of the time the unit will be displaying your GPS rather then your sonar. Running a split-screen with a larger GPS and smaller sonar is how I do it.
Locating the structure/fish/bait is done at the console. I think he has both the 998 and 1198 located there.