In my opinion the reason behind this is due to students' hesitation and nervousness. Moreover, due to the peer fear and false prestige also the cause of passive participation in exploring themselves.
I wonder if it is also because we might be measuring learning against an irrelevant standard such as UK or US Englishes. They would have very different idioms, don't they? Unless students appreciate their own voices, they may have to live in perpetual fear of ridicule, hence the hesitation and nervousness. Are the students hesitant and nervous in other classes (such as Math)?