Permanent revenue system of Lord Cornwallis proved oppressive to the tenants because they were

The Permanent Settlement introduced by Lord Cornwallis fixed the revenue demand that zamindars had to pay to the British. Since this amount was fixed regardless of actual collection, zamindars exploited tenants ruthlessly to maximize profits, often charging more than the actual land revenue to ensure their own income. The system placed no limits on what zamindars could extract from peasants.