Why be that specific? The WHO’s guidelines already made it clear why some names may be skipped. Referring to Xi Jinping directly would be counter-productive to their goal of depoliticising the name.
Are you really asking why we expect the WHO to be transparent and honest? Because they are an institution that has been given a lot of trust and power. Because their attempts to shape a narrative can harm important efforts like accountability for the virus’s origins and early reporting/containment (or lack thereof). Because people don’t like being lied to or propagandized by overt attempts to shape public opinion. Because an allegedly scientific institution should not play political games.