They have been warned 6 hours before the strike on 'Ain Al-Assad by Iraqi Prime Minister, 'leaked' from Iran. They want to avoid needless bloodshed because inside the base there weren't only US troops stationed there, but also innocent bystanders--soldiers from other countries. Iranian Brigadier General Amir Ali Hajizadeh said, the retaliatory attack is not for inflicting as many human casualties as possible, but to cripple main US infrastructure inside that base and give warning to Pentagon. Seems that they don't heed the warning though.. such is the morals of the corrupt tyrants.