The reason is because he uses blockchain technology. Blockchain can be said to be the server of every bitcoin transaction and it is very decentralized. No one can break into blockchain. Why ? because that is an impossible thing to do.
You almost got the point here, but you forget to talk about double spends, the cherry on the blockchain cake is the way it manage the double spend transactions, if some one spend the same coin to multiple address,only one will confirm and let the blockchain keep growing without erros, that's why bitcoin is the safest system. Some banks already have a double spend problems and the bad way of manage it makes them lose tons of money.