Allah Has clearly ordered us in Quran ( Sura 59, Verse 7) to follow the Prophet (SAWA) , and the Prophet (SAWA) never prostrated on carpet but only on the clay or earth and said ( Earth was made for me as a place of prostration and purification) جُعِلَت ليَ الأرضُ مسجدا وَطَهررا)
We Shia, prostrate on the clay (Turba) simply because we follow the Prophet (SAWA).
Wassalam.
Wassalam.