Because the superoxide releases the most energy when formed, the superoxide is preferentially formed for the larger alkali metals where the more complex anions are not polarised.
bɪkˈʌz ðə sˌuːpɚɹˈɑːksaɪd ɹᵻlˈiːsᵻz ðə mˈoʊst ˈɛnɚdʒi wɛn fˈɔːɹmd, ðə sˌuːpɚɹˈɑːksaɪd ɪz pɹˌɛfɚɹˈɛnʃəli fˈɔːɹmd fɚðə lˈɑːɹdʒɚɹ ˈælkəlˌaɪ mˈɛɾəlz wˌɛɹ ðə mˈoːɹ kˈɑːmplɛks ˈænaɪənz ɑːɹ nˌɑːt pˈoʊlɚɹˌaɪzd.