This may be an example of a question wrongly put.

Historians often take the position of cultural relativism to avoid passing judgment based on their own biases, in theory this means that their work will only include facts and any conclusions they draw will be interesting and useful regardless of who read them. If they wrote from the perspective that their moral system is the correct moral system then their work would be useless, as it would be mostly opinion and not historical fact.

They embrace cultural relativism because it's the only way to do the job properly.