Previous prophets were sent to a particular group of people and their message was supposed to be followed for a particular time period. Since previous revelations were time-bound, God did not preserve them Himself.

the Quran cannot be compared with the previous Scriptures. The Quran is unique in many aspects. It is the final revelation to mankind. Previous scriptures were sent to their respective people only. The Quran is universal.

So God promised to protect the Quran

God says in the Qur’an
(9) Indeed, it is We who sent down the message [i.e., the Qur’ān], and indeed, We will be its guardian.

Muslims accept the original unaltered Torah (as revealed to Prophet Moses, peace be upon him). Muslims also accept the original unaltered Bible (as revealed to Prophet Jesus, peace be upon him). These original scriptures were revealed by God (Allah).
none of these scriptures exist today in their original form or in their entirety.

https://www.youtube.com/watch?v=sjNOmCg9At4

https://www.youtube.com/watch?v=sYE27mDxzaY