Why are hashes base16 (hex) encoded rather than base58 encoded?
Mark H's answer is correct, but I think there is a more fundamental point: addresses are directly intended for human consumption. Transaction hashes (as well as public keys, block hashes, raw scripts, raw transactions, ...) are not. They are sometimes visualized, but this is primarily for debugging purposes. And hexadecimal is a very natural choice for debugging, as it's easy to see which bytes correspond to which characters.