In order to fully understand this, we have to consider why physical copy prices fall so rapidly in the first place.
That part of the equation belongs to retailers. Publishers would actually love to see prices stay higher for longer but for retailers, shelf space is money. Backroom storage is money. Both are limited and both prioritize product that sells well over product that does not. That's why they want a 2 year old game up and gone so that they can make room for what comes next. Inventory management is a massive part of large-scale retail.
Digital doesn't have that issue, at least not intrinsically. There is literally no reason to pore over your digital inventory on a daily basis in order to free up room for next week's releases. Therefore, price cuts are done by schedule and not necessity, which means they're done at much longer intervals.
What I'd really like to see would be a cheaper starting price for digital but the roadblock there is the worry of retailer reaction. If one publisher began doing that and no one else joined in, there would be some risk involved. Of course, if all publishers started doing it, that risk would be mitigated, but they can't legally plan out something like that.
