بسم الله الرحمن الرحيم
This is prohibited because it is imitating the way shaitan eats as is narrated by Ibn Umar where he said Allah’s Messenger -sallahu alayhi wa salaam- said, “whenever one of you eats then he (must) eat with his right hand and whenever he drinks he must drink with his right hand, indeed shaitan eats and drinks with his left.” Collected by Muslim.
Translated by Najeeb Al Anjelesi
Source: المنظار في بيان كثير من الأخطاء الشائعة