What they know about English is limited to some basic English grammar and vocabulary. They have very poor pronunciation and intonation, with a strong local accent, and they cannot speak English fluently. For them the grammar-translation method is the most acceptable because they can basically teach English in Chinese. English teaching is comparatively new in our country and most English teachers, especially the more experienced ones, know little about the newer methodologies. Quite often, ironically, it is those who can't teach English well who are chosen to teach the methodology courses.
Some teachers never hear about other teaching methods, only the grammar-translation method.
The most serious fact is that this inefficient approach is continually reinforced. When some of the students who have been taught with the grammar-translation method turn out to be English teachers, they are most likely to use the same method in their teaching. When my student teachers had their teaching practice in the middle schools in the countryside, their mentors would advise them to teach English in Chinese. Their students felt very happy if they did so. When my colleagues don't use the grammar-translation method and try to give our students more oral practice in class, some students grumble and some even request a change of lecturer.