Well if you compare say the R22 (relatively new design) with an older design, say a B47, you will see the newer design puts "engine on" performance above engine off performance.
A narrower, smaller,lighter blade is generally easier to turn under power and hence maximise performance. (Especially if you have a small engine).
Generally customers will want better performance figures when the machine is working normally, over good auto rotational performance when the machine goes wrong.
With two engines especially (where the machine is supposed to go wrong less often), carrying around all that extra fuel and the spare engine, a light inertia rotor system leads to better sales / performance figures.
Compare a B212 rotor with an EC135 and you will see what I mean.