Very simple and very easy point. Shanks and Rayleigh know where one piece is and what it is. The only reason neither one of them want to go get it is because
•Shanks - wants Luffy to find it. He is already a Yonko and he doesn't want the problems that come with being Pirate King. If you look at it Shanks is a very laid back guy. He like to drink and no one messes with him. If he became the next pirate king everyone would be trying to kill him. Also it could be a sign of respect for Gol D. Rodgers.
Rayleigh- I have no idea why he doesn't go get it