Ukraine banned yandex in 2017 for mass data harvesting of Ukrainians, probably that's the main reason why gp did not find it in Ukrainian Bing. It's still possible and perfectly legal to access yandex or other data harvesting engines/disinfo websites over VPN or Tor, so the ban main objective is to protect non-tech-savvy users from the interference of hostile foreign criminals.
Not sure why the downvotes, as, yes, Yandex along with several other russian internet service companies is blocked in Ukraine and it is almost certainly why it is not being set as default, because doing so would straight up break search altogether.