Why is speaking in tongues only mentioned in 1 Corinthians and not in Paul's other letters?
Speaking in tongues is mentioned only in 1 Corinthians because the issue was unique to that carnal, fleshly congregation. Paul addressed it in response to their specific letter's questions. Other churches—Philippi, Ephesus, Colossae, Rome—had no such problem, so Paul never mentioned it to them. The context presents it as a local problem, not a norm for all churches.
Les explains that Paul was answering the Corinthians' letter point by point, using "now concerning" as he responded to their questions. The whole idea of tongues in particular had caused a problem, and it was unique to the Corinthian church. Les notes that never again in all of Paul's other churches is it so much as mentioned, only to the church at Corinth. The Philippians were not aware of it, the Ephesian church had nothing to do with it, and neither did the Colossian church, Rome, or anywhere else—not a one. Only to this carnal, fleshly congregation did Paul have to address what we call tongues. The context then returns to 1 Corinthians chapter 12, where Paul deals with the Body of Christ, composed of people baptized by the Holy Spirit. That Holy Spirit baptism is not an emotional or physical phenomenon; it is unfeeling, and believers know it happened only because the Book says so.