One of the problems with warfarin is that there is a lot of variability between patients. The main clearance enzyme for warfarin, CYP2C9, has reduced function in around roughly 25% of patients due to genetic polymorphisms. The target for warfarin, VKORC1, is highly variable due to genetics and the substrate concentrations, vitamin K, can vary greatly with dietary intake. Warfarin also has a narrow therapeutic window. If concentrations are too high there can be bleeding problems and at concentrations too low it's ineffective.
The end result is that the initial warfarin dosing can be extremely variable and requires close therapeutic monitoring when starting a new patient on the treatment. It's a far cry from something like aspirin where there is a pretty good idea of what dose a patient will need. A drug company producing something similar to warfarin that wasn't subject to such high genetic variability would be clinically preferred due to lower health care costs in starting treatment on new patients. That said, when you're properly dosed warfarin is pretty damn effective.