ðˈɛɹfoːɹ ðeɪ stˈɑːɹɾᵻd sˈeɪɪŋ "sˈɪpi" æz ɐ mˈɑːɹk ʌv ɪnfˈɔːɹməl ɐfˈɛkʃən ænd ˈɔːlsoʊ bɪkˈʌz ɪt wʌz ˈiːziɚɹ ɔnðɪ ˈɪŋɡlɪʃ tˈʌŋ.
Therefore they started saying "Sippy" as a mark of informal affection and also because it was easier on the English tongue.