Why do different perspectives in psychology use different methods?

Psychology is a diverse and multi-perspective discipline due in part to its origins in biology, physiology and medicine. Because our behaviour is determined by different factors, such as our personal characteristics, our social environment, our cultural environment and our genetic endowment, psychologists have had to adopt a large number of approaches to its study.  The dominance of different methods and theories has also been influenced by historical events and cultural shifts in the way knowledge is gained and data evaluated….(short extract)