The correct answer to this open question is the following.
Although there are no options attached. we can say the following.
Some of the factors that might have facilitated Chinggis Khan's conquest of the East could have been these.
We can say that an important factor was the overconfidence of Sultan Ala ad-Din Muhammad II who believed he was more powerful than Genghis Khan.
The context was that Khan had sent diplomats and traders to different regions of the East to increase trade for the Mongols. These people were arrested by the troops of the Sultan and this represented a big insult to Genghis Khan. That is why he decided to send his army and conquer the perpetrators.
Khan wanted revenge and imposed his power and control in many Middle East territories.