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There are five reasons for this which i listed below.
1:- Unlike centralized ( concentrate (control of an activity or organization) under a single authority) fiat payment systems, Bitcoin is fully open-source (The term "open source" refers to something people can modify and share because its design is publicly accessible) and decentralized.
2:- second reason is that because Transactions can be verified independently when ever you want at any time.
3:- Bitcoin or btc payments can be made instantly and directly without an mediator (a person who acts as a link between people in order to try and bring about an agreement; a mediator)
4:- The whole system is protected by a combination of elliptic curve(In mathematics, an elliptic curve is a plane algebraic curve defined by an equation of the form. that is non-singular; that is, it has no cusps or self-intersections) cryptography and hashing on the sha256 curve.
5:-Together these mechanisms is sufficiently provide large enough random key-space that cannot be attacked by any hacker or gamed through mathematics