I've been to China and taught English for a couple weeks in which they have the same difficulties of linguistics in common with the Japanese. In fact, what I could realize is that the problem is chiefly rooted in their native language and its semantic and theoric foundations. The way they learn a language since childhood is basically by characters and tones that are quite incomparable with the way we learn Persian here in Iran. You should contemplate that our mother tongue is essentially an Indo-European language which falls in with language branches such as Anatolian, Indo-Iranian, Greek, Italic, Celtic, Germanic and so on and so forth. So, as you can see, the majority of people in these countries learn and speak English far better than the Eastern Asians. 

In my opinion, this must be the essential reason for their difficulties in learning English. In truth, Chinese and Japanese are categorized under The Sino-Tibetan languages, in a few sources also known as Trans-Himalayan, are a family of more than 400 languages spoken in East Asia, Southeast Asia, and South Asia.