nobody knows why because this is a decision that the creator (Satoshi) made and to my knowledge there is no comment from him saying why.
however, it probably has something to do with the fact that hashes are interpreted as "numbers" (uint256), for example a block header hash is interpreted as a number to compare it with the target for the difficulty calculation purpose. and in writing we write numbers in big endian notation hence the endian-swap for representing them in GUI and using the non-reversed in code under the hood.
probably since uint256 is used elsewhere too (such as representation of a transaction ID) that reversing effect also happens there.