Jap DLsite does not accept some overseas credit cards.
I've called two different banks before and they both tell me that the problem is that some Japanese bank refuses some overseas credit card transactions, they don't know the reason either.
They leads into the "easier to buy" point. If a game is good and is accessible (easy to buy, helps if localized / synchronized launch ...etc) then more people will buy it.
I basically pirate games that:
1) I cannot buy (because the bank refuse my money)
2) It's a game I wouldn't have played if I had to pay for it (example:
most CG only rpgmaker games with some rare exceptions), or otherwise an artist I do not want to support.
It's the same with many gamers, before things like Steam and itune equivalent became available, I pirate most of my games because its the easiest and most accessible way to get it. Once Steam, GoG ...etc is there, my game library on both have become huge

I haven't pirated a normal game in a long time.