The first and best reason is because one of these methods has real power and the other doesn’t.
Let me explain.
Holy water is a sacramental which means it is a “sacred sign” which bears resemblance to the sacraments. As the Catechism states, “they signify effects, particularly of a spiritual nature, which are obtained through the intercession of the Church. By them men are disposed to receive the chief effect of the sacraments, and various occasions in life are rendered holy.” (CCC No. 1667)
In other words, all of the power of God and His Church is behind the use of this blessed water, which is why it has such a long and storied history, particularly in the area of banishing demons who are extraordinarily averse to even the smallest drop of this water (and this is by their own admission to exorcists, by the way).
As this article states, “We are told that, through the prayers of the Church, by the pious use of holy water, the intellect is enlightened, and the will moved from evil while being prompted to do good; and both body and mind are thereby strengthened and healed.”
Compare this to the waving of burnt sage in a home supposedly to “cleanse” it of “bad vibrations.” Also known as smudging, it is nothing more than an ancient superstition that has no real evidence of being effective. There is plenty of anecdotal “evidence” (so do rabbits feet, for that matter) but it lacks the one thing holy water has that saging doesn’t – a confirmation from Satan himself.
As for the “evil vibes” – this is a construct of New Age thinking which declines to recognize the personification of evil.
Evil is not a vibe or a negative energy. It’s a person.
“We must realize that we face an... evil that is a person,” said Pope Paul VI in a General Audience in November of 1972. “Thus, we know that this dark and disturbing being truly exists and is still active with predatory cunning. He is the hidden enemy who sows error, misfortune, decadence and degradation in human history. . . . He is the evil and cunning charmer who knows how to infiltrate everyone's individual psychology. He finds the open door and comes in: through our senses, our imagination, and our concupiscence . . .”
This definition is considerably more realistic than that of New Age theorists who fancy evil as being nothing more than an irksome vibe. Adopting this naive view of evil is dangerously opposed to objective truth which offers us centuries of historical proof (both secular and religious) of the existence of a diabolical being who is only controlled by a God-man known as Jesus Christ.
Having explained all this, it becomes obvious that the only way to rid a home of evil is to use something that has been proven to work - holy water.
But remember, regardless of what is used to banish him, the devil will not stay gone for long if the occupant of the house is consorting with demons in any way, either through direct contact with the occult and other pagan practices, or with attachment to a life of sin.