Why didn’t people speak up about the Clintons’ criminality? Why didn’t people speak up about Weinstein? Fear, loss of face, loss of position and power. In Stillington’s case, he could have ended up in prison before execution (although the Plantgenets weren’t into murdering clergy or women; the Tudors brought that innovation). But definitely prison.
He spoke up when it was safe to do so.
Stillington shouldn't have had any fear of stepping forward with the knowledge of Talbot having been married to Edward when she married Butler in 1449. Edward hadn't met or married Elizabeth yet. He wasn't even King until 1461, and would have been only 7 at the time Talbot married Butler.