maybe because a lot of the potential people you might want to donate actually use paypal?
there's already monero and bitcoin donation addresses there, but paypal is something most people already know about and can deal with (from multiple currencies around the globe).
I've downloaded a couple of hundreds of euros last year to various open source and Internet advocacy projects, and I've always used PayPal to do so.
I love the projects I've donated to, but not enough to trust them all not to fuck up storing my banking info. This way, I have all my donations easily available via one interface that I don't use for much else.
there's already monero and bitcoin donation addresses there, but paypal is something most people already know about and can deal with (from multiple currencies around the globe).