Because deconstructing something as merely a social/ideological/cultural construction, without any necessary bearing to reality is classic Marxism. Race, gender, sexuality, class structures exist as ideological constructions; not because of biology, or how biology interacts with the environment. This is because Marxism is predicated on the idea people are what they are solely because of the environment and acculturation, an idea descending through enlightenment thinkers from the outdated pre-genetic scientific theory of tabula rasa and beyond. The focus is on diversity because the next step in Marxist thinking, where things are only ideological construction, is to address the inequity arising from them. This next step is the deconstruction of false ideological precepts which create inequity. Race/gender diversity means dissembling the dominant hegemonic force; straight, and white (the bourgeois). The only reason a Marxist would have in calling something a social construct is to then dismantle the "winning" construction.