Nature of Confession
Confession by nature is a “Personal Encounter” with Jesus through the ministry of a priest. Emails and Phones remove that physical aspect of this personal encounter which must be present for one to validly receive the Sacrament of Penance.
He (Christ) personally addresses every sinner: “My son, your sins are forgiven.” He is the physician tending each one of the sick who need him to cure them. He raises them up and reintegrates them into fraternal communion. Personal confession is thus the form most expressive of reconciliation with God and with the Church. – CCC 1484
Another reason is secrecy. A priest is bound by a special oath (Seal of the Confessional) to keep everything a penitent says to him in the confessional absolutely secret. A priest may never reveal any detail of a penitents sin to anyone, not even civil authorities, even under the threat of their own death. Breaking the oath would lead to automatic excommunication (latae sententiae), the lifting of which is reserved to the Pope himself (Code of Canon Law, 1388 §1). Phones, email, etc are never secret and can always leak details of a penitents sins to third parties.
The Pontifical Council on Social Communications on The Church and Internet says:
Virtual reality is no substitute for the Real Presence of Christ in the Eucharist, the sacramental reality of the other sacraments, and shared worship in a flesh-and-blood human community. There are no sacraments on the Internet; and even the religious experiences possible there by the grace of God are insufficient apart from real-world interaction with other persons of faith.(9)