It took decades for the technology to develope to accurately test.
Piltdown was spotted immediately as a false interpretation. Elaborate hoaxes are rather rare in science, so when additional specimens were found, people tended to believe Dawson, the finder. Having Catholic monk along for one of the finds added to his credibility.
De Chardin remained silent on Piltdown for forty years, never mentioning in his writings what should have been the highlight of his career — assuming he didn’t know it was a hoax.
The hoax ruined the careers of several people who trusted them.