This isn't an accurate representation of what happened. Masks and other forms of PPE (personal protective equipment) were in limited supply during the start of the pandemic. PPE is critical for healthcare workers and scientists to do their jobs in a safe and effective manner. The public was told not to use and buy PPE for that reason (at least from healthcare experts... normal news sources and social media are bad at explaining this stuff correctly). Once the supply of PPE increased, the public was given more access and recommendations for mask use.

Regarding masks as a preventative measure for COVID-19, there just wasn't much information on how effective they really were at the start. Scientific research takes time and the answers aren't always clear/complete. We "change our mind" as we gather and interpret more data. The (over)generalized consensus went from "the effectiveness of masks at stopping COVID-19 is inconclusive but you still shouldn't buy a bunch of masks because we need those to keep healthcare workers/scientists alive" to "masks (normal, cloth, and other materials included) can slow/reduce the spread of COVID-19 and should be worn when in public... but please don't hoard since supplies are still limited".