The bottom line is visual appeal. Each brand offers a different look, but some bats are clearly better looking than others. Rarity - Like most collectibles, bats are judged by rarity. For example, a 1950's Mickey Mantle game-used bat is far more rare than a Mantle bat from the 1960's. The same can be said for other players such as Willie Mays and Hank Aaron.
Rarity can have a major effect on pricing.
For example, Monte Irvin bats are virtually impossible to find, therefore, the rarity is reflected in the price. Even though Frank Robinson was a superior player to Irvin, his bats are more common so the difference in price is apparent. In addition, modern bats are manufactured at a much higher rate than they were in the past. For example, Ken Griffey Jr. has had more bats made in two years than Mantle received in his entire career. On the other hand, some modern players are very protective of their equipment.