iron-based alloys over aluminum in structural members of bridges and buildings?
Because it's lighter with about the same strength as iron alloys
Aluminum is chosen in aerospace because it is light weight. Iron is preferred in structures here on earth because of its strength and suitability to endure gravity for a long period of time.
Aluminum generally has a better strength-to-weight-ratio. Steel alloys are still used in some of the parts of a typical aircraft, where tensile strength and toughness are the major considerations, over weight. The landing gear, bolts and pins, hinges, bearings, for example. Steel is used for bridges and buildings, because it's a lot *cheaper* than aluminum, and saving weight isn't nearly as critical as it is in aircraft. Steel is stiffer than aluminum, and It's easier and cheaper to make reliable welds in steel as well. Welding aluminum tends to be time consuming and expensive.
Strength-to-weight ratio.
