The only book before Quran is Torah. Injeel and Zabur are were never revealed as books. The teachings of Deen or shariah of Torah are more or less preserved. The rest is inzaar for which prophets were coming.

Since the prophets were coming there was no need to preserve the text. If there were any confusion the prophets were there to correct it.

At the time of Quran God decided to end the prophethood therefore it was necessary to preserve the last book with all the details necessary as there will not be any prophet to clear a confusion.

To end the prophethood at this time was appropriate because the world has entered into the era of globalization and history was being recorded unlike previous times when history was not well recorded.