Reasons for late discovery of noble gases:
1) They are incapable of reacting with other elements. Being inert, they are not present in a combined form with other elements.
2) They are minor components of air, that is, present in only trace amount. A Brief note about noble gases which causes their late discovery: The noble gases (historically also the inert gases) make up a group of chemical elements with similar properties; under standard conditions,they are all odorless, colorless, monoatomic gases with very low chemical reactivity. The six noble gases that occur naturally are helium(He), neon(Ne), argon(At), krypton(Kr), xenon(Xe), and the radioactive radon(RN). Oganessons(Og) is variously predicted to be a noble gas as well or to break the trend due to relativistic effects chemistry has not yet been investigated.