Apoloniusz Wrzosek
Apoloniusz Wrzosek voted up Don Barzini's answer

Pigs live in filth, eat anything including filth, and are therefore considered unclean. Any amount of time spent around live pigs leaves no doubt about this logic. 

I had a Muslim friend tell me once that pigs are physically incapable of looking up toward heaven, so are considered too lowly, beneath Allah's grace.