No one asked for immediate withdrawl of ATM-desposited checks. And while the paper checks themselves may be "antiquated", the routing and account numbers make for conveniant electronic processing. It makes sense to hold the check until the request for the money can be sent to the bank. That should be the next business day. With the account information on the check, there's no reason it can't be processed over wire.
Which it is.
So why hold a check for up to 6 days?