It is for the same reason that no second language teaching method can be the best one. However, within an individual's brain, certain regions may respond well to the acquisition of the lexicon, others to the perception or production of sounds, others to the formal learning of grammar, and still others to learning through conversational interaction. A learner may be better at some of these skills than others, and the right method will be the one that allows him to learn best, i.e. the method to which his brain most readily responds. A method which presents material in a way that fails to target efficiently a learner's neural circuits to subserve the skill being taught will not be the right method for that brain. Once again a teacher can provide general guidelines, but no guaranteed approach.
Brains respond differently.
Another reason why there can be no right method is because post-sensitive period learning is also more dependent on motivation. In the same way that recovering from neural damage requires dedication to the therapeutic regimen, an adult second language learner has to find the instructional program that works for him, and he must stick to it for the lengthy period of time needed to become proficient. Of course, the learner's needs will change with time and therefore pedagogical needs will also change. The right method at one time may not be the right method at another because the learner's brain will have changed. An approach, method, or technique that does not resonate with the learner's neural systems will frustrate achievement and will demotivate.