It's common sense, that's why! (6)
I believe the answer is:
reason
I believe this clue is a double definition.
'it's common sense' is the first definition.
(thesaurus)
'why' is the second definition.
(thesaurus)
'that's' acts as a link.
(Other definitions for reason that I've seen before include "Incentive" , "Motive; argue" , "Justification or motive" , "'Motivation, grounds (6)'" , "Explanation; logic" .)