In all likelihood, YPN doesn't have anywhere near the number of advertisers that Adsense has. With four or five link unit categories and up to ten links per category page, that's up to 40-50 advertisers per Adsense link unit.
Many times, the Adsense link unit advertisers are way off-target. People here often complain (and rightfully so) about YPN ads being off-target. If you think that conventional YPN ads are off-target, I think you would see YPN link units being much more so.
I would be surprised if we see YPN link units anytime soon. I'd love to be proven wrong, because that is my preferred ad type.