There's a mandate to cover the curriculum. The District focuses on learning the basics and on boosting standardized test scores. Project goals do not match goals associated with standardized achievement tests. It is difficult to communicate to parents just what students are doing and learning. It's cumbersome to use Project Based Learning in large classrooms or with younger students.
Boys are less willing than girls to talk freely in groups.
Girls are less willing than boys to take charge of building products. Students, especially younger students, can get lost in the doing of the project and forget the project's purpose. It is difficult to have long-duration projects with very young students. It is difficult to define goals for a project. Valid assessment is complex and difficult.