The mongols and a couple other barbs need to spawn inside borders
In the case of Keshiks, we need some pressure on Hungary, Poland, Lithuania and Bulgaria too for historical accuracy
If Kiev is strong enough, it would prevent these things to happen, or at least postpone them dramatically
Also, time is an important factor
It takes at least 15-20 turns for the mongols to cross the eastern part of europe and reach Poland, Lithuania and Hungary. That's at least 50 years
If there aren't any spawn points near those countries, the timeframe would be completely unrealistic either for these countries, or for Kiev