What makes the knee so stiff after total knee replacement?

Scar tissue. After a knee replacement, achieving maximum knee motion is often a race against your bodies natural healing process. Though your pre-operative range of motion can ofter predict your maximum expected post-op range of motion, aggressive physical therapy is crucial to achieve the best result.
Swelling, scar. Early on, swelling from blood or fluid production, along with the tissue trauma makes a knee feel stiff. After 6-12 weeks, scar tissue formation can be a major cause.
Scar tissue. Initially, inflammation after the surgery will cause the stiffness. As inflammation subsides, the new tissue can be immature and fibrotic. Early intervention of soft tissue mobilization and joint mobilization is key to decrease the level of stiffness in the new joint. I believe physical therapy should be initiated as soon as possible, ideally the next day.