William met most resistance in the South East soon after the Battle of Hastings.  As he was normally in London, William could quickly stop any rebellion.

Only when King William was away from London did South East rebels have the chance to start anything.

The first act of resistance in England after 1066 was in Kent, based in Dover.  Rebels appealed to Eustace of Boulogne.  They tried to attack Dover castle, but failed.  Eustace fled in disgrace.

By building castles, William made sure he could deal with any rebellions.  He faced no major resistance in the South East after 1067.