Paying for services is different from donating to a large entity where you have no guarantee of where your money goes. Paying Microsoft for Office products for example I know I'm getting Office, aka what I'm paying for. In the case of tech based non-profits you have no idea where your money ends up. Unless you're a billionaire funding a new "part" of a non-profit or donating specifically to MDN, other people who can only donate say $50 might not be so lucky.

Edit: This also makes me wonder if there's non-profits that work on tech that aren't throwing their money away beyond the scope of the project. The moment non-profits are doing more than just paying engineers and other essential employees / volunteers I begin to worry a bit.