Why? Different material?
Aluminum isn’t new in fighter jets. The F-15 and F-18 both make fairly extensive use of aluminum, IIRC. My impression is that the material isn’t the issue here, it’s the grade and construction. The popular opinion at the time of the ATF competition was that the then YF-22 was selected because it was cheaper and had thrust vectoring, while the YF-23 was more expensive and didn’t have thrust vectoring at the time of the competition even though plans were on the table to add that particular capability. So the Air Force cheaped out on a plane that eventually was produced in very limited numbers, and now we get this story about corrosion—well, I’m not surprised.