The Aggressive chassis was built mainly for a single purpose. Actually, it wasn't a completely new chassis, just a different belly pan, hood, and a few other cosmetic pieces. The reason for building it was to update styling and to fit the triples with triple pipes under the hood. The Evolved styling, as far as I know, didn't have enough room under the hood for triple pipes so they designed the Aggressive styling to take on that role. Since they were limited mainly to the high end, high HP models, that's the reason I'd imagine why you see such a limited amount of them. Both styles were eventually replaced / updated with the Gen2 styling.
Breaking free from the thoughts in my mind