Why do behavorists object to schools of psychology that study consciousness?

Behaviorists, like John B. Watson- for example, object to schools of psychology that study consciousness because they believe that "consciousness is a private event that is known only to the individual... [and] if psychology was to be a natural sciences, like physics or chemistry, it must be limited to observable, measurable events- that is, to behavior." (14).

Source:

Rathus, Spencer. Psychology Principles in Practice. Austin: Holt, Rinehart, and Winston, 1998. 14. Print.