Libra is stable coin and bitcoin is not and I guess you must also have read the reason that why Bitcoin is not. Bitcoin is decentralized and this is not the demerit or shortfall of bitcoin it was created to be decentralized. Libra on the other hand is governed by a single agency which means it's just another model of ripple which is popularized by Facebook. It's backed by securities and debt securities? I don't know what do they mean with this because a cryptocurrencies is mined and not created by keeping a backing debt security. I think libra is just a digital currency with the securities backing however it's far away to be called a cryptocurrency because this is absolutely not how cryptos behave.