Why were noble gases discovered late?
The noble gasses are inert or unreactive due to the fact that they all follow the rule of octet and have complete
Because most elements were discovered through their reactivity with other elements, typically with oxygen, it was difficult for scientists to work with a substance that seemed to have little or no chemical properties based upon the lack of reactivity.
I hope this was helpful.
Very good historical presentation on this website: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_18%3A_The_Noble_Gases/1Group_18%3A_Properties_of_Nobel_Gases