It's likely coinbase was chosen simply because it's the easiest way for their victims to buy coins. There is a vulnerability with Pizza businesses in that they trust the caller is making a legit order, and don't ask for pre-payment before delivery thus leaving them open to fraud. Whoever is doing this is clearly bluffing and hoping just a handful of businesses pay the extortion and likely flooded the mail system with these letters. I bet the payment address is the same for each extortion letter meaning they have no way of knowing if a company pays or not.