Since Bitcoin is decentralized, there is no standard or anyone to dictate what it should be. Nonetheless, "BTC" has been the generally accepted notation for Bitcoin from the early days of Bitcoin.
"XBT" is a new abbreviation for Bitcoin that is starting to come into use and reflects its growing legitimacy as an international currency.
XBT comes from an International Standards Organization (ISO) that keeps a list of currencies. If a currency is not associated with a country, then the standards say that is should start with an “X.” For example, gold is XAU.