I guess it is just a relict of an ancient pronunciation. In West Germanic the initial k was certainly still labialized. In OHG, e.g., you find spellings with qu-.
Fater unser, thû thâr bist in himile,
sî giheilagôt thîn namo,
queme thîn rîhhi,
sî thîn uuillo,
sô her in himile ist, sô sî her in erdu, ..
Early 9th century.