The Wonders of Technology

To answer your question, it's probably because PayPal is generally a hell of a lot cheaper to plop PP onto the site than it is to get hosed by the big boys (are you listening, NPG?), who try to lock you into a bundled service in order to get a merch account.

As one of the "little boys" who has set up credit card donations for candidates, I think it bears mentioning how the costs of taking donations directly into one's own account has come down to be affordable for any candidate running a serious campaign.

NPG is a waste for local candidates, but that's not anywhere near the only option available.

Well for god's sake, son, send me the info! ;)