The CDC reversed course on April 3, citing new studies showing that asymptomatic people can transmit the virus to others without showing symptoms.
“In light of this new evidence, CDC recommends wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain (e.g., grocery stores and pharmacies) especially in areas of significant community-based transmission,” the agency said.
It's the fact asymptomatic people can spread it (which wasn't previously known regarding this specific virus) that was the reason the recommendations changed.
Link: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover-guidance.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fcoronavirus%2F2019-ncov%2Fprevent-getting-sick%2Fcloth-face-cover.html