The Quran is the word of Allah as well as being the book of Allah while the Torah and Injeel are books of Allah, Speech is an attribute of Allah that is eternal, perfect, and nothing can be changed in it, therefore ghair Allah cannot tamper with the Kalam of Allah. plus the promise of Allah about the Quran to be protected and the responsibilty of the rabbis/priests in protecting their books as mentioned by sister Bintamirah is the main reason. Torah and Injeel were for a specific time period and for specific nations, that is why they were not protected as was the Will of Allah, Quran as you can see calls upon the whole of mankind and is for all times, therefore it will be protected by Allah. P.S Shias believe Quran is created, convenient isnt it? P.P.S If I have unknowingly pitched myself in the first sentence from athari to maturidi or maturidi to salafi then someone correct me as I have little knowledge of this stuff