Ask for money. If this model works for them (it seems to), then there's no reason to change it. Also, by having "sponsors" you open yourself up to "change this article to be like this, or we'll pull our support from Wikipedia." This may seem like a far-fetched scenario, but unless you only use the ad revenue for extra income and don't depend on it at all , you are vulnerable (no matter how many levels removed you are…
> Also, by having "sponsors" you open yourself up to "change this article to be like this, or we'll pull our support from Wikipedia." Why can't the people who donate money do the same thing--that is threaten to stop donating unless an article is changed to their liking?
There are also many more readers of Wikipedia than there are advertisers. This makes gathering donations a more distributed means of paying their expenses.