The basic reason? To date, the B16A1, the B18C1, and the H22A_ (I think 4), are all sought after by aftermarket tuners because they are VTEC. The B20Z2 is non-VTEC. So, coupled with supply and demand (as you said), because the engine is non-VTEC, it literally costs a fraction of the cost of the other engines you mentioned. But it's simpler tuning does of course permit the engine to make all that low end torque, which supercharged will certainly get you up to speed very quickly.. that is not to say that it won't fly naturally aspirated - because it will.