Why would Jesus not save the pigs with the man?
Everything that Jesus did was done with purpose.
Under the Old Covenant, was an unclean animal (Deuteronomy 14:8).
In the New Testament, they are illustrated as sin (2 Peter 2:22)
Jesus explained with the illustration of the pigs that those who are unclean (being in sin) will be used and then terminated by the enemy of our souls.
2 Timothy 2:26
Then they may come to their senses and escape the Devil’s trap, having been captured by him to do his will.
Another lesson that Jesus revealed was that with humanity, being created in God’s image means more to Him than animals. Jesus protected the man from being killed by the demons but allowed them to go into the pigs to make it crystal clear how much He loves us.
The demons called themselves Legion, think about that for a moment. A Roman legion was comprised of 5000 men. Demons hate humanity so much that they would go to any length if necessary to destroy us.
Another reason that many demons went into the man was to make it harder for an exorcist to cast it out, being stronger in numbers. They wanted to make sure that their assignment would be completed but they did not put Jesus in their equation.
Those who put animals above people have a love that’s twisted. We must be kind to animals but never in any circumstances choose an animal over a person.