Jewish teaching is that Shabbat services are for thanks and praise, not for petition [see Berachot Yerushalmi]. That's why the rabbis cut out the middle 13 blessings of the daily Amidah.
But we still petition God on Shabbat: Hashkivenu, Oseh shalom, Mi shebberach, prayers for the government and for Israel, etc. So why did we not go all the way? What is the dividing line?
[Sephardim cut out the middle part of Hashkivenu (Vehagen baadenu...), but curiously leave the rest (ufros alenu...)]