In my opinion facebook wants to make more money and thats why they launch Libra. Imaging if they will use bitcoin, the transaction fees will not go to them. They created the stable coin libra so they have full control. There is no mining in Libra coin nor it has a transaction fees but facebook will have all the profit when they have the majority of the coins at their disposal. So at the end it will just end up in profit, but at the same time the merchants who hate volatility will really welcome their coin.