This is a partial list. Please add more reasons below as comments if you think of any.
1. When we use English a lot, over the course of the year, the students’ minds get weak. They also get confused, because they don’t know what language they are in.
2. The proof is in the pudding. A class taught at 98%-100% in the target language all year, including all classroom instructions in the target language, processes in L2 a lot better and faster than one that has heard even 10% English over the course of the year.
3. If we stop in class to give a pep talk in English about how we don’t want to hear any English, we appear to our students as hypocrites. (We can always do that kind of processing during the metacognition part of class, during the last five minutes).
4. When we use only the target language, we go slower and we speak much more clearly. We have to, because we can’t rely on a quick translation to clarify any confusion in class, which is a major no-no in our work. We cannot use English to leap over mud puddles in our work, we have to wade through them, but we have to do so deftly, so that we don’t get bogged down in the mud.
5. Staying only in L2 lowers the stress level in class. No matter what happens, the kids know what the teacher will be doing: speaking in the target language. This consistency is good for kids who may have grown up with all sorts of inconsistencies in many of their other classes and are, as a result, often on a kind of edge in class, wondering what is supposed to happen next. They may be sitting there acting as if they understand because we require that, but not really following (they always understand far less than we think), and then in that little moment where we start using English, some of them sense that something is different. Up and down go their emotions and focus in class.