Why does bitcoin use AES-256-CBC? Why not some other block cipher operation mode? Is there a specific reason?
Encryption isn't part of Bitcoin at all. Nothing on the blockchain is encrypted. There is no encryption in consensus or the P2P network.
Only wallets have encryption for protecting private keys. Bitcoin Core specifically uses AES-256-CBC because it is not known to be insecure and is widely used. Other wallets may choose to use other encryption schemes.