The point being that the I6 was the highest level performance engine through then (L20-24, RB) with the exception of the DR30s.
Ohh maybe I misunderstood, do you mean is that why the VG30 and SR20 have been ditched? well the SR20 was lost really only because of emissions, although I can't work out why they chose not to do more with the SR20VET. The VG30 is from what I understand a fairly direct ancestor of the VQ series which are still used today.