Ten years ago when the story begins, it's possible that Shanks isn't already a Yonkou. He is a rising star among pirates for sure, as Whitebeard ackownleged later, but maybe not much more. He has already sailed around Grand Line once onboard Roger's ship, then had his own share of adventure : maybe he and his men want to relax and enjoy the world for a bit. Shanks is portrayed as a carefree man throughout the series, after all, so he would definitely be the kind of person to let things be for a couple of years
Another hypothesis (theory, even, as I don't have any solid facts to back it) is that Shanks has already been to Raftel and knows what lies there; and so, he isn't in a hurry to return to that island, because he knows somehow that the conditions for anyone to reach it aren't yet satisfied.
A more prosaic factor might be that Oda hadn't thoroughly planned all this New World and Yonkous business so early in the series (the first few chapters tell a much more straightforward tale, with almost naive simplicity at times). He didn't give it much thought because it didn't matter back then and maybe we will simply never know.
Last edited by Seafarer; October 04, 2013 at 06:07 PM.