Why are third parties are not using bitcoin biockchian or blockchain to store customers data?
Because not everyone want's a decentralized database system. Bitcoin is decentralized and nobody would like to store secret information on a public ledger.
If they use it to store customers data, information will not be leaked? Like bitcoin I assumed YES.
The type of data matters here. In case of bitcoin, the transactions are hashed and stored on the blockchain in a tree like structure.
Information cannot be leaked because it is already publicly open for everybody to verify.
A decentralized blockchain can only be used when users need to verify the credibility of that information.
Or are the third parties affraid because they want to be always centralized?
For storing critical/secret information it is absolutely necessary for the information to be centralized and stored securely.
In case of bitcoin the private keys are stored by the user itself. This makes us in control of the information.
This secret information is not stored on the blockchain. We only use it to broadcast information (transactions) to the blockchain.
What is the remedy to customer data leakage?
Storing it securely by encrypting it and keeping it offline is best way in my knowledge to prevent data leakage.