Rowhani's aides have said he proposed an accord in 2005 with then-French President Jacques Chirac to allow uranium enrichment in exchange for the highest level of monitoring by the U.N.'s nuclear watchdog agency. The deal did not gain support from other countries such as Britain and the U.S.
Why does the author of the above article prefer to use the word watchdog instead of watchman? Is there any notable difference between the meaning of these two words in this context? Which one sounds more formal and correct in this context?