Mendeleev's version of the periodic table does not have noble (inert) gases. Why do you think the noble gases were late in being discovered?
Well, how reactive are the Noble Gases?
Chemistry is driven by the observation of chemical reactivity, by the analysis of how matter transforms in chemical reaction.
Of course, given inert materials, there is no chemical reactivity to observe. A few compounds of the Noble Gases are known now, i.e. xenon oxides, and fluorides, but these are the preserve of the specialist, and notably they are compounds of the most reactive elements, oxygen and fluorine.
Given this lack of reactivity, this chemical inertness, it is unsurprising that the Noble Gases were identified relatively late.
Note that the inclusion of the inert gases in the Periodic Table demonstrates the relationship between reactivity and electronic structure. The Noble Gases have a full valence shell: thus they are (i) difficult to oxidize, and (ii) difficult to reduce.