Do not cast pearls before swine / holy things to dogs
Here’s what it means
Do not keep offering sacred truth in a way that invites hardened mockery, trampling, or violent abuse.
Why the Bible says it this way
The writer uses a picture to explain one thing by describing it as another.
A helpful detail: Jesus is not denying the gospel to whole ethnic groups. The animal images describe a hostile response and call for discernment.