Why teachers sometimes use Spanish to teach English?

My teacher answered that they sometimes speak Spanish because they wanna speed things up a little bit.
When students are exposed to a new language for first time, it it is really difficult to understand without knowing anything about that language.
From my point of view, it is completely true. However in Chile 10th or 12th graders have been exposed to English for many years and they continue using Spanish in the classes. It happens because most of the students do not like English due to the fact that they think that it is not useful. So teachers have make a great effort to motivate their students to learn English in an interesting way obviously.
Another reason is that some teachers have very low expectations about their students. They think that the students are going to fail because they are bad at science, maths, etc.