Thing is, with Paypal it's "too easy" to claim a refund.
Let's be honest; some of ED's DLC is pretty shonky and allowing Paypal transactions would probably result in FDev losing a significant number of sales due to refunds.
On top of that (and probably more problematic), you're going to get the "test pilots" who buy stuff, use it for a little while and then claim a refund when they're bored of it.
Sure, the fact that we've got the whole Money>Arx>stuff thing going on, as well, makes it even more complicated but, from FDev's POV, that's just going to make it even more complicated to remove DLC when a player reverses a transaction.
I tend to be quite forgiving of DLC but, I gotta say, if I'd used Paypal to buy the Arx which I used to buy the "Celeste" COVAS, I would have requested a refund even if that meant I lost a bunch of other DLC purchased with those Arx.