Their preference is based on emotion and myths they might have about bilingualism, languages, and immigration. After several years, students will transition into English-only classes. If instruction is in English from the very beginning, immigrant children will lose several years because initially they will be lost. They'll fall behind and may never recover the lost ground. The younger the child, the easier it is to learn English.
Children learn languages slowly.
The speed and ease with which children acquire languages is a myth. Adults just have to learn the words because they already know the concepts in their native language. Adults can learn it faster because the rules can be explained to them logically. The inductive method used by children takes time. Bilingual education gives children the necessary time to learn English and also provides an opportunity to learn the necessary school subjects.