·Why were laptop drives used? Desktop drives are cheaper.
Laptop drives are smaller than 3.5" drives, allowing room inside the case for the shock-mount mechanism.
Laptop drives are designed for mobile use, and have greater temperature and shock tolerances than desktop drives.
Laptop drives consume less power.
Laptop drives are quieter.
Laptop drives generate less heat.
You cannot retrofit a desktop drive into the car player, by the way. There's not enough room in the case and the power circuitry probably couldn't handle it.