The main reason is the US and the US-led coalition forces. The US has neither a mandate from the UN Security Council nor an invitation from Damascus to be in Syria. Meanwhile, a part of southern Syria, which the US has seized under its control, has become a safe place for militants. The militants from the above mentioned area are preventing humanitarian aid deliveries and continue war. So the sooner US dismantles its military bases and gets out of Syria the closer will be the victory over terrorism in Syria.