Do you know why the two plastics are different? I would naively expect that everything that would work as the inner layer in tetrapak would also work in cans (cans deform less). Is it that making the layer stick to tetrapak is simpler/different than making it stick to metal?

I'm not a material scientist, but the two are constructed very differently.

Tetrapaks are laminated, meaning that various layers of solid materials are glued together. Think of it as a sticker on something.

For cans, the epoxy is a liquid coating that's applied by spraying or roller coating, more like paint.

And, for the cans, it then cures in place. The BPA is like a catalyst and doesn't bind with the material, which is what allows it to leach out so well.

Polyethylene, in addition to being a safer molecule, uses catalysts which don't end up in the final product, which means you can't be exposed to them.