It's not that non-vegetarian is forbidden, but it is written in scriptures that to achieve moksha, vegetarian food is required because vegetarian = non-violence and non-vegetarian indirectly supports violence towards animals. To achieve moksha, one cannot be violent towards any species, including animals.
Also, the requirements for each varna/caste is different. Brahmins and Vaishyas are not allowed to eat non-vegetarian whereas it's ok for Kshatriyas and Shudras to eat non-vegetarian since they fight in wars or do physical labor which requires that extra strength.
However, regardless of caste, if one wants to achieve moksha, non-vegetarian food is forbidden.