They're playing kid sister and dragging up the rear. They don't have the numbers. If you've got the numbers targeting becomes extremely easier to do, you have such a wider base to draw from. The more ads you have - the easier it is to match them to your target audience. YPN doesn't have that. Adsense has it in spades and it's one of the key components of their success.
When MSN rolls out it won't have it either. Everybody's playing catch-up to Adsense right now and unless somebody comes up with something totally revolutionary and totally new it just ain't gonna happen. Massive influx of advertiser's dollars.. that's what's needed. Yahoo can't generate that. MSN can't generate that. Google OWNS contextual advertising and they probably will for some time to come.