Why was pork forbidden because pigs eat filth, yet chicken was not forbidden even though chickens eat it too?

The Muslim submits to the command of Allah and His Messenger, whether he knows the reasons for the rulings, their wisdom, and their benefits or not. Eating pork due to its filth is not the sole reason for its prohibition. Even if this were the only known reason, it is not permissible to prohibit what Allah has made lawful due to the possibility of other unknown reasons. As for chicken that eats filth, it falls under the ruling of al-Jallalah.