We must first realize that many school districts assign material to be taught at certain times. It is often the case that grammar, sentence structure, development of paragraphs, essay writing, etc. are directed to be taught for a given semester, while literature is to be taught another semester in the same academic year. It may be the decision of the director of curriculum or of the principal of the individual school, or by the superintendent of the district, or even of the elected school board of the district which makes such decisions. In other words, the individual teacher may not have a personal choice in the matter.
Just as in any other subject, a given teacher has personal preferences and individual tastes. Many teachers enjoy diagraming sentences themselves, so will have a greater interest in teaching the same to their students. Similarly, a given instructor may not enjoy grammar on the personal level, as much as reading and comprehending literature. Therefore, it should come as no surprise that such a teacher would be more enthusiastic about teaching literature.
Please realize, also, that there are English teachers who may not have received the best possible education, themselves, in given areas of language study. If the teacher experienced a lack of education in a given study during their own educational process, that lack of knowledge is likely to be passed on to that teacher's own students.
For example, an instructor may have been an excellent student in the study of grammar, but may have had a personal struggle with understanding
Shakespeare. Obviously, that teacher will be unlikely to spend as much time trying to teach "A Midsummer Night's Dream", say, than teaching how to write a good paragraph.
Any teacher's preference in subject matter is always going to be influenced by his or her own areas of personal enjoyment and level of skill.