I think corps are only just beginning to explore the range of visual possibilities unencumbered by years of living their corps-specific "brand", based their use of [mostly] military-derived uniforms. Unsurprisingly, these uniforms, as well as the military roots from which the activity evolved, informed the predominant style of movement: highly-regimented marching.
As the color guard activity evolved over the past 30-40 years - where once-taboo dance and themed costuming have now become standard - these same elements have gradually migrated into the visual repertoire of hornline and drumline, from simple horn and body moves at key impact points to the near-constant, highly-coordinated sequences we often see today.
So where are we now? Now that several corps are eschewing the overarching "brand" of their corps proper uniform for attire tied to each year's specific production, brass and drums - like the guard before them - are no longer constrained to move in the same fashion as before. There is fast becoming a breakdown in the lines between guard, hornline, and drumline where the whole ensemble can contribute visually, in-character, to the show. This seems to be what is rewarded by judges today.
And to your point - why is this difficult...why is this excellent? Well...why are there technical and lyric components to a brass audition? Why do we crave the ebb and flow of a gorgeous ballad followed by the raucous aggression of a highly-technical percussion feature? The diversity of programming creates interest and intrigue. Opening up the brass and drummmers to such possibilities demonstrates a broader range of skills that - when properly applied and mastered - raises the possible level of excellence.