Because it’s 2 volumes against 19....Still amazing feat from DS though
But there’s a huge difference between a new series and an old one, people who want to read OP most likely already did, and not many are going to buy and get on the ride of a ~100 vol series, how many people are actually buying the old vol of OP in 2020?
DS is relatively new and it just ended, naturally people are flooding in to buy the entire set.
But people posting these charts won’t tell you guys any of these background info because they just want to shit on OP.
Of course, 99.99999% chance Demon Slayer is going to be the best selling manga this year, no denying that, not to mention it already broke OP’s record of highest sale a year and half year from Oricon charts back in 2011