Why does Libra need to be centralized?

It is because of scale and speed.Decentralisation makes the system less vulnerable to hacks or shutdowns, but takes time: the Bitcoin blockchain, for instance, can only process about seven payments per second.By comparison, the centralised Visa payment network can support up to 24,000 payments per second.Initially, Libra should be able to handle about 1,000 transactions per second.The white paper says that, over the next five years, Libra will shift from the current proposed model—also called “permissioned” blockchain—to a totally decentralised or “permissionless” blockchain.Of course, there is no guarantee that will ever happen.