That is my question. If it's already narrow? How it is already narrow?
As others posted previously; because Christ is the only Way. Always was and will be.
Matthew 3:2-3 “Repent, for the kingdom of heaven has come near!” For this is the one who was spoken about by the prophet Isaiah, saying,“The voice of one crying out in the wilderness,‘Prepare the way of the Lord,make his paths straight.’”
Matthew 7:13-14 “Enter through the narrow gate, because broad is the gate and spacious is the road that leads to destruction, and there are many who enter through it, because narrow is the gate and constricted is the road that leads to life, and there are few who find it!
Matthew 21:32 For John came to you [Pharisees] in the way of righteousness and you did not believe him, but the tax collectors and the prostitutes did believe him. And when you saw it, you did not even change your minds later so as to believe in him.
John 14:6-7 Jesus said to him, “I am the way, and the truth, and the life. No one comes to the Father except through me. If you had known me, you would have known my Father also. From now on you know him and have seen him.”
This narrow Road/Gate/Way point of Jesus' is found in all four Gospels. The early Christians, who were almost exclusively converted Jews, who were working hard to maintain their way (yet converted in a flash from it to another way), actually called their Christ following (non-sacrificing) group "The Way" based on this point. Only later did the term "Christians" come into use. That Jesus Christ is The Way to salvation and it's narrow,
always was and will be, is extremely fundamental to Christianity. Really. If anything anybody teaches you conflicts with The Way being narrow, jump ship.
Saul, for example, wasn't on this narrow way until he met The Way, 1/2 way on the Damascus way. But Jesus put him on His Way anyway.