7075 aluminum uses Zinc and magnesium in the alloy. The more common and much cheaper 6061 aluminum uses copper and silicon in it’s alloy.  In simple terms, 7075 aluminum can withstand 58.4% more pressure than 6061 aluminum. It’s also more expensive to produce and machine, which is why a lot of companies use 6061 instead of the more expensive 7075. This article from Kloeckner Metals goes into great detail on the exact differences.