Yahoo is looking at the same marketshare numbers, so that actually can't be the reason.
It certainly can be the reason that Google's offer isn't as strong -- the value of default search place in the third-place-with-continuing-declining-share browser is conceivably quite different to a party that owns what is, depending on which measure you look at, either the first- or second-place browser and a party that doesn't have any browser.
Either it's scaling effects (owning a few percent makes you more known, which drives new users) or Yahoo is better at selling ads through those users. Note that in the former case, there's even more incentive for Google to block. And if the second is true, oh boy, they certainly have a problem at G...