If Paul mentions baptism for the dead why don’t other Christian denominations practice it? 

From what I could find it sounds like other Christian churches don’t interpret Paul’s statement as an endorsement of the practice. Instead they believe he simply used it as an illustration that others have hope of resurrection through Christ and therefore even do baptisms on behalf of those who are dead (Source). 

The Roman Catholic Jerome Biblical Commentary confirms that the Christians in Corinth “would undergo baptism in the name of their deceased non-Christian relatives and friends, hoping that this vicarious baptism might assure them a share in the redemption of Christ.” – Brown, Jerome Biblical Commentary, 2:273

We know through Latter-day revelation that this was an approved practice and ordinance that was restored in the last days (Source).

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