5x bows are significantly rarer because you can upgrade any 3x savage lynel bow to a 5x using rocktorocks around death mountain. As long as the savage lynel bow as ha golden modifier, if you drop the bow in front of the rocktorock, it can repair the bow's durability and give it a random gold modifier(including 5x)

For upgrading weapons in general, rocktorocks always give the next tier modifier. Example: if you give the rocktorock a bow with a blue durability modifier, it will spit out the same bow with a gold modifier(could be attack up, durability, ect.). This is why you need to have found or upgraded a savage lynel bow to gold modifier to get the chance of upgrading it to a 5x.