im glad u asked this question,lot of nonbeleivers have this misconception....now Allah did sent his message everytime,and preserving tht message was a responsibility given to man,here after man failed repeatedly...note:book did not fail at tht time, but man did fail in preserving the book..now look at god's perspective,naturally u may ask why did he send so many messages knowing men would fail to preserve them,and later send the qur'an..answer: god has sent his message for every community for every tribe at diff points of time(meant for only particular group of people and for tht time period only) but obviously there wasnt any 'globalisation' by man at those times for him to spread the message very fast....if u ask me true 'globalisation' began 1400yrs ago.......if u happen to know history,coincidentally the spread of human race frm continent to continent began some 1400 yrs ago also,here when man travels,he spreads the message,it makes sense to send qur'an as the final message........proof of qur'an not failing is given in the qur'an itself by god giving a promisehimself. "We have, without doubt, sent down the Message, and We will assuredly guard it [from corruption]" (Al-Hijr 15:9)