I think for the next few years cost will still be a major factor in the specifications of a car. Part of the beauty of the DP01 was, overall, it was cheaper to run (and fix!) than the previous Lola (and I'm pretty sure cheaper than the IRL cars now). If / when the series returns to it's "glory" days (and there's simply more money in the series), then it can be opened up to additional chassis manufacturers, etc. But until then (and I'm afraid it will take some time), they'll need to keep the focus on costs (w/o sacrificing safety or performance, of course).