Something to do with it adding more "player data" to "carry around with you", increasing either the size of the network packets or the amount of database traffic. Whereas they have already fully accounted for every inventory slot.

They often say "why can't you just delete old currencies?"

But they also admitted that in Japan, they don't place sentimental value on items like other countries do. For example, having an award cupboard. So often people will hold onto books, items and gear just because of the work that went into getting them.