It'll be for two reasons:
1) because they need to finish the game but can't get the people in the studio to do it.
Releases go right up to the wire, frequently sorting last minute patches, and they just can't have the manpower available to do it.
2) The physical release is also pretty important for sales. Everyone online likes to think that digital only is fine, but there's a fuckload of people that will still go to the shops to buy it, and Sony/Naughty Dog rightly don't want their hard work to get released and only have half the userbase they're aiming for. Digital sales may have overtaken physical last year but it's a 53/47 split. Anyone out there want to jeopardise 47% of their income? Nope, nope, nope. I do buy some games digital but for a big release I'd always buy a physical copy (resale value if nothing else) and hence the delay.