Why is prothrombin time test not used for heparin
Thats because the mechanism of action is different between warfarin and heparin.
Warfarin inhibits the formation of prothrombin (also known as factor II) by inhibiting the metabolization of vitamin K (for extensive details have a look at the Wikipedia article about Warfarin).

Heparin inhibits the blood coagulation by binding to antithrombin III which functions as a inactivator of active clotting factors as factors V, IX, X, XI and XII. It also binds calcium which is important for the blood coagulation. For more details see the Wikipedia under Heparin.

Therefore Heparin interferes with the formation of Thrombin and the test does not work. A nice image of the coagulation cascade can be found here, which also shows the points where Heparin interferes. The test can be done with Batroxobin, see here.