Same boat here. Had a few of those in the same week. One of them won an auction and throw an absolute fit that I don't accept money orders or cash and insisted I was in the wrong even though it clearly states in bold letters on my listings that I only accept Paypal. Ah well.
As to the question on why someone doesn't take cash or money orders, I can only give my own opinion, but it is way too easy for a buyer to say they sent the cash (even if they didn't). Not to mention that if the cash actually did get lost in the mail (or stolen or whatever), that buyer will always think of the seller as a thief. There's just nothing good about doing cash through the mail. Risks aren't worth it for either side. On the money order front, just too many fakes out there, too many places that won't cash them (my understanding is even Wal-Mart won't cash the money orders that they sell - I could be wrong, but that's what I hear), and the bank fees are crazy if it bounces. My bank charges either $19 or $26 now (I forget what they recently raised it to, it was one of those two prices) if the money order is fake, bogus, or worthless. Not worth it in any respect for me, so all I take is Paypal.
And really, I don't see why someone wouldn't have Paypal or wouldn't be able to sign up and use it unless they are under 18, at which point they legally can't buy on Ebay anyway, so that's a moot point.