Why do some Christians oppose Tarot cards? The title should explain it all.  Many (perhaps most) Christians view Tarot cards as evil, or at least something to be avoided.  Why?
Tarot cards are widely used for fortune telling and other occult practices, which most Christians consider to be sinful. While the cards may not be themselves considered bad, or possessing them inherently sinful, it is likely to be thought of as an indication that you are involved with occult practices, or are laying yourself open to temptation in that regard.

Having a good reason for possessing Tarot cards, not involved with occult practices, is going to be looked on differently. The Tarot deck is in fact just a standard card deck from the 15th century, and only came to be associated with occultism in the 18C. There are a number of historical games (some of which I play) that use the deck, and have no occult connections at all.