^Might have something to do with corrosion, it's the only thing I can think of. By making the bolts and screws out of a less noble metal than the supports themselves, the supports won't corrode before they are no longer into contact with the less noble metals in the screws. While this makes the screws a lot more prone to corrosion, they are a lot easier and cheaper to replace than the supports themselves (which are probably a PIA to fix). And if you have enough redundant supporting, a faulty screw or two won't be a problem.
Though, I see a few logical fallacies here myself, so this is not likely to be the correct answer. For starters, the same effect can be obtained by simply strapping a bar of a less noble metal directly on the metal support itself. This "sacrificial anode" will prevent the corrosion of any nobler metal it is in contact with. It's also a LOT easier to maintain and replace than thousands of individual screws and bolts high up in the structure.
Yeah, I just wrote two paragraphs to say "I have no idea".
