Nadal's withdrawal doesn't surprise me at all. I don't see him coming back until Monte Carlo though he has put down his name in several tournaments before then, but there will be some last minute withdrawals as well.
I agree with the statement that Nadal doesn't want to see any defeats on hard courts hence the opt out. I don't know if anyone remembers other than Nadal fans, but Nadal's performance was abysmal on hard courts after his return and had a series of losses. He did not win any titles upon return in that late part of 2009 and the first part of 2010. He lost to Del Potro at the USO and then to Murray at the AO. In fact, since some of you are so busy putting asterisks next to some of the winners', you might as well put one next to Del Potro's USO trophy. Because Nadal had just made a comeback after seeing a defeat to Soderling on his favourte surface at RG and skipping Wimbledon as a result as his confidence was shattered by then, he wasn't exactly riding high on the confidence level and so a player like Del Potro had no problems brushing Nadal aside in straight sets - a player who hasn't seen a win over Nadal since then.
Nadal only won his first title in 2010 once the clay season rolled around. He sealed the deal at MC once again and went on a tear and won the next 3 Slams. One can also put an asterisk next to Nadal's 2010 USO, where he basically had to face no solid hard court players and Nole was off from a very long and hard win over Federer in the semi and was mostly done by then (Andy went out early with a wrist injury). In fact, I'd like to put another asterisk next to Nadal's 2012 RG as Nole was half done after the semi-final, as he had to beat Federer in the semi and he didn't like his defeat the year before to the same player, so he had to give his all in that match vs Federer. So Nadal benefited from there as well.
I am very sure this is all Nadal's decision. He wants to come back strong and so if there's any doubt in his head, he's not going to do it. This, of course, doesn't necessarily mean that history will repeat again. They do sometimes but not always or else we'd be leaving in a repeated world. Best of luck to Nadal nonetheless. Tennis does miss him.