I think his age is what Brumdog66 is referring to.
Bernal is 23, Roglic is 30 (31 in October).
If we subscribe to the old cycling truth, that your best years are 26 to 32, it is a fair statement that Bernal has not yet hit his peak, while Roglic is nearing the end of his peak years.
In Roglics defence, I'd say his longevity may be more than his age, because he got a late start in the sport, much as is the case with Fuglsang, so Roglic may be great for 5-6 years more.
If that happens, we're in for a hell of a Tour battle the next few years, as we add Pogacar, Evenepoel and a healthy Sivakov to the list of real contenders