why? cause Rocks is above Roger and Whitebeard?
how
Because we were told so.
Sengoku said Rocks was Roger's strongest enemy and in that volume published during the month-long break It was said again.
Now, if this will be somehow disproven at GV, things can change, but for now It doesn't seem so.
Also, It fits.
Harald Is the strongest king ever of the strongest nation. Wano (<Erbaf) had his strongest fighter in Ryuma; what's the problem if Harald, who's has even and higher standing, Is stronger than Roger or WB and on par with Xebec?
Not to mention we know Giants have a role to play in the big events of the world. We saw that in the Harley.