Diversity is an interesting concept. We don't call a collection of people with different types of fingernails "diverse." To value diversity is to value bridging social divisions that are often tools of domination, power.
A commitment to diversity doesn't just lead to better outcomes, more creative solutions and more resilience in the face of challenges.
Diversity also fades, as the divisions become less effective tools of exploitation.
I can see why it's threatening to some.