Loans get paid back... donations don't. An organization that takes donations (like wikipedia) will have to keep asking for donations... while an organization that has a debt can take the time to figure out a revenue model not driven by ads to allow them to repay the debt.
Wikipedia's average donation is tiny. That's why they keep asking. The way you've written this it suggests that if instead of giving wikipedia a $2 donation, we have them $2 no-interest loans, weed be helping them prepare for the future and they wouldn't have to ask for more donations (or loans) from us. That's not how it works.
