The fact that Walt led them to the money. What's Walt gonna say? "I have no idea where those barrels of money with my fingerprints all over them that I led you to came from"? Like I said in an earlier post, real life drugs busts are based on much flimsier evidence than the kingpin leading you to millions in cash. All you need is something to build a case around, you can then add Jesse's confession, testimonies from Hank and Gomie, maybe evidence from Heull, investigations into the car wash, following up on other leads (the two guys Walt ran over, Brock's poisoning, etc.) The millions in cash isn't the be all and end all of the case, it's just undeniable physical evidence to base the case around.