They pay like 1.54 USD per 25 minutes, which is quite low. I guess that teaching and explaining for 25 minutes worth more than 1.54 USD.
Some of the students are mean to the teacher, they will give you a bad rate. It's not easy to explain something to a student who doesn't talk the same language as you.
Students chose lessons which are not appropriate for them, they chose advanced level and they don't understand the content so they feel like the teacher is not explaining everything easily, you get bad rate at the end.