Among the reasons is that one of the major motivations for lying is deficiency. People generally lie for reasons such as fear, greed, trying to gain an advantage, or not wanting to hurt someone's feelings (which is often really just fear, such as fear of losing a friendship or a spouse). Because Allah does not have deficiencies, needs, or fears, Allah has no motivation to lie. 

Truth is also an inherent quality of the divine and I don't think there is any culture that has envisioned the absolute divine power having untruth. Some polytheistic cultures may envision demigods lying  but not the actual underlying creative power of the universe. (As there are many religions throughout history and today, someone is welcome to correct that point, but it is my general sense.)

Our inner nature also points to the idea that truth is ideal, and lying is a falsehood/deficiency, because any human of sound mind - even if they are the most evil dictator - will like to know the truth and detest being lied to, and this is an acknowledgment of the divine value of truth within us. 

I recommend reading Shi'i hadith such as in al-Kafi regarding the attributes of Allah. However there are some books on this website that also cover the subject that may be of interest.

Happy reading!