Why did Henry VIII execute Robert Aske and Sir Robert Constable after granting them pardons for their involvement in the Pilgrimage of Grace?

In December 1536, Henry VIII promised to pardon the rebels who had taken part in the Pilgrimage of Grace, the great northern rebellion that had caused him so much trouble that autumn. Robert Aske, one of the rebellion’s principal leaders, was even invited to court, where he met the king and discussed the grievances of the people of the north. Yet, just seven months later, Aske was executed at York, and Sir Robert Constable, another prominent participant in the rebellion, was hanged in chains at Hull.

So what had happened to Henry VIII’s promise of mercy?