There wasn't an off the shelf alloy to do it at the time, nor were there presses that big.

Automakers over the decades have turned to making cars from catalogs of parts.

Teala due to lack of suppliers early on (few top tier suppliers wanted to work with Tesla) learned to just make what they want if it isn't available or costs too much, so making an alloy and finding someone to build a new kind of press for them isn't too far fetched.

So it probably came down to too many new things and processes to do so, and no appetite for the risk involved.

Now that it's been proven to work, it's safer to follow

My guess would be a mixture of all of those combined with the cost of retrofitting entire production lines vs retooling existing processes for updated models. Tesla built their factory from the ground up, so they had a massive capital investment regardless of whether they used traditional methods or "Giga Press" technology (TM)

Probably technical challenges, technology development hadn't made it possible yet, and very high cost of equipment.

Automakers only started using aluminum in the past few years.