@Captain:
Mmm, they wouldn't want to cannibalise their own sales, same reason things like the colour version and the logs keep a little way behind what's current. It's incredible that a simultaneous physical/digital release would have such an impact on One Piece specifically though. Just seems a bit weird that it's the only series getting this treatment.
Other series don't sell 3 million copies every volume.
Now that Naruto , Bleach, AssClass, etc. have all finished the closest thing they have is HxH selling 1 million copies… and that's only a volume once a year. My Hero Academia is on the rise and all its volumes combined are selling pretty decent, but its not individually supporting the entire company. Haikyuu, Food Wars, etc. all sell okay. But nothing anywhere near the beast that is OP.
OP is the only huge seller they have right now (and its by far and away the hugest) so yes, it gets special treatment.