You should perhaps remember that TI is a very international product and concept, and so rather than looking at who is the ideal customer, I would ask myself what personality traits would someone need to benefit from TI.
To illustrate my point, I would imagine a lot of teachers and lecturers in the UK with a general interest in fitness would find TI challenging and inspiring and a great topic of conversation at dinner parties, but they are often underpaid with respect to their academic qualifications.
Similarly, different cultures across nations prepare people to accept and look for the wisdom of others at earlier or later ages in life. I would imagine in India or Japan an average 18-20 year old would have the wisdom and modesty to listen to the advice found in self coached materials such as TI whereas in many european countries, we believe we can reinvent the wheel by ourselves until later in life.
For me a TI customer needs a good educational background, self discipline, a willingness to cast aside all they thought to be previously true, focus and patience.
I am not in the least surprised that TI is popular in Japan.