as salam alaikum
the Qur'an is a miracle for past, present and future generations and its being in form of a book makes it reachable to every human being. Of course it must be understood and that is why knowledge has been made compulsory in Islam.
Other miracles would have been witnessed only by a group of people. Also, in modern times could have been falsified by the use of technology. On the contrary a book divinely preserved and passed down both orally and scripturally generation after generation do not present such problems.
Similarly, the crucial hadiths necessary for salvation have been passed down through divine providence and tawatur.
With prayers for your success.