Answer to Question 1
As David Berliner has argued, schools and classrooms differ from each other in a number of ways, including quality and quantity of personnel, teaching methods, budget, leadership, and community support. In addition, cognitive and effective outcomes of schooling are affected by complex interactions among such factors as student characteristics, teacher characteristics, curriculum materials, socioeconomic status of the community, and peer influences. The result of all this complexity is that teaching methods and programs that work the same way for a broad sample of teachers and students in different school districts are few and far between.
Answer to Question 2
Behavior is complex and generally has myriad causes. By examining only one or two aspects of behavior at a time under controlled conditions, the researcher can tease apart these causal factors.