I take it this is your first time selling BTC at Coinbase ?
The wait is normal. I have had to wait longer many times. Never given a reason why (or any answer for that matter) when I put in a support ticket but eventually the money turns up in my account.
The wait is usually external to Coinbase. It's a delay caused by the outmoded and primitive ACH (Automated Clearing House) system of electronic transfers used by U.S. banks. To be fair, other countries use even more shitty systems. Something like three business days is usually about right for how long these transfers take.
I had one Coinbase transaction take one day longer than they said it was going to. Every other transaction, though, came in earlier than they said it would. In one case, I had it the next day. The ETA they give you when you make the cashout is almost always accurate, in my experience, and usually faster than they said.
I speak only of BTC --> USD transactions. I generally sell rather than buy BTC.