Nike Inc.’s top human-resources executive told employees Wednesday that the sportswear giant “has failed to gain traction” in hiring and promoting women and minorities, weeks after complaints of inappropriate workplace behavior led to a leadership shake-up.

In a memo to staff reviewed by The Wall Street Journal, Monique Matheson wrote that the company wants “to create a culture of true inclusion. As part of our plan, we need to improve representation of women and people of color.”

“While...