Because he only needs to win the USO and the next Australian (if not Wimbledon) and by this time next year the record might be in the bag.
Nadal will probably win the FO and Wimbledon and has a good shot at the US open. If not him, there are a couple of other guys who could take slams.
At the moment, I wouldn't be so sure. Nadal and Federer are beatable in other tournaments, but at Grand Slams they're not really being challenged.
Djokovic and Murray are the main challengers, but only on good days and even then if they can keep their cool. Murray has yet to win a GS and I don't know if Djokovic can mentally handle being public enemy number one.