This lowered reactivity is due to the relativistic stabilisation of ununennium's valence electron, increasing ununennium's first ionisation energy and decreasing the metallic and ionic radii; this effect is already seen for francium.
ðɪs lˈoʊɚd ɹɪɐktˈɪvᵻɾi ɪz dˈuː tə ðə ɹˌɛləɾɪvˈɪstɪk stˌeɪbɪləzˈeɪʃən ʌv ʌnˌʌnɛnˈɪəmz vˈeɪləns ᵻlˈɛktɹɑːn, ɪŋkɹˈiːsɪŋ ʌnˌʌnɛnˈɪəmz fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒi ænd dˈiːkɹiːsɪŋ ðə mətˈælɪk ænd aɪˈɑːnɪk ɹˈeɪdɪˌaɪ; ðɪs ɪfˈɛkt ɪz ɔːlɹˌɛdi sˈiːn fɔːɹ fɹˈænsiəm.