Why Use Aluminum for a TT Platter?

it is relatively easy to machine and is inexpensive. However, aluminum is poorly damped and subject to resonance. So it was imperative that other materials are used such as steel,lead,bronze and many others to numerous.I am guessing also the availability and easy to work with then say acrylics...I am sure their are more reasons to using it as a chosen metal.

Maybe because it's non-magnetic and so won't suck the magnet in the phono cartridge down into it. Just surmising here. Stainless steel would be more expensive to buy and machine, glass is freqently used, as are the plastics. But I don't know that all of those don't come with their own issues. I dislike the sound of plastics in particular.

i would think Copper would be good. especially an alloy that is tougher (so it will not bend if dropped)
The problem of the tarnish could be overcome with plating or a coating. A highly polished (then coated) copper platter with a lead interior (more mass toward the outer rim)... Mmmm.
Or composite sandwiches. I guess it is money. Stuff like this costs a lot to make. Easier/cheaper to make a four inch thick acrylic than a one inch thick copper alloy.

I was under the impression the weight of the platter was more for maintaining a steady constant speed thru inertia so the motor is less taxed in the quotient.
I never understood aluminum either, is it not a quality of the platter to allow the vibrations to be drained and not reflected back at the stylus interfearing with its purpose that one would want.

Many people call any platter not metal, acrylic which is not a true description. The platters infused with carbon and nylon known as Derlan are as the manufacturers claim closer to that of the vynil itself.
Maybe your question is really the answer to what material has not been tried for a record mat and why.
Cheers