Because strait is the gate, and narrow is the way, which leads unto life, and few there be that find it. Matthew 7:14
Lord Jesus said the gate to life is strait, and the road to it is narrow. Why?
It is because there is only One Way to Life, and all the rest (which are many) can only lead to destruction. Similarly, there is only One True God, and all the many others who call themselves gods are merely deceivers.
Which Way is the Way to Life?
In John 14:6, Jesus told us clearly that He is the only way to life.
Jesus said unto him, I am the way, the truth, and the life. No man comes unto the Father, but by me. John 14:6
What and Where is the Gate?
In John 10:7-9, Jesus also called Himself Gate of the sheep, and only those who enter through Him can be saved. Unfortunately, there are very few who has truly found Him.
Then said Jesus unto them again, Verily, verily, I say unto you, I am the gate of the sheep. All that ever came before me are thieves and robbers: but the sheep did not hear them. I am the gate; by me if any man enter in, he shall be saved, and shall go in and out, and find pasture. John 10:7-9