EC-7s aesthetic design likely requires more material.
ARC-8 is modelled after the ancient BBS RC wheel design from the 90's. Folks w/ dual street/track aspirations likely wanted an updated look - ala M3 competition wheels.
In general, I believe the more concave a wheel, the more material your need - for a given stress rating, material and manufacturing process.
I'm no wheel engineer - just assembling my observations.