The 1997 Uncirculated $5 Jackie Robinson Coin had a mintage of only 5,174. The Proof Coin had a mintage of 24,072. The were both produced at he West Point MintThe term
Uncirculated usually means that the coin has never been circulated, it is a coin that is in “Mint State.” This means the coin is just as minted, with absolutely no wear on it at all (although bag marks are acceptable.) However, a coin that is a “Proof” is made with special dyes and processed differently. They usually shine and have a mirror-like finish.
More people, naturally purchased the Proof version of the Jackie Robinson gold coin and less of the Uncirculated or Mint State.
Right now if you purchase either of the coins on Ebay or any other online source you will probably pay more for the Uncirculated vs the Proof coin. This sounds backwards but it is because of the rarity of the Uncirculated coin due to the low mintage.
In addition you will notice that coins that have been graded by a professional grading service will sell for a much higher premium. I have never sold or even had one of the gold coins but check out a graded Jackie Robinson Gold Uncirculated coin that recently sold on Ebay.