The story of the Golden Fleece starts in a small town called Iolcos in Greece. Pelias, son of Poseidon and the half brother of Jason's father, Aeson, became the king of Iolcos when Aeson was considered the rightful ruler. Because of that, an oracle showed up to Pelias and warned him that a descendant of Aeson would have his revenge. After Jason was born, Pelias decided to send him to a mission which would end up with Jason's death.

Now the question is why Colchis. Well, Colchis is the farthest point East in the Black Sea and the region at the time (at least 10th century BC) was not familiar to Greeks, though they did establish some short-lived colonies centuries later. One could speculate that the reason Jason was sent to Colchis was because the area was dangerous since it was inhabited by many "uncivilized" tribes such as the Heniochi, Zygii, Svans (or Soanes in ancient sources), Zans etc. but you could counter that by saying that Scythian lands were far wilder and more dangerous than the somewhat centralised kigdom of Colchis. If the story is a myth, we can come up with many possible explanations but we have to take into consideration that etymologically, the word "myth" means a tale or a story. Myths aren't necessarily made up, though of course they are almost always exaggerated. I think it is more plausible that a similar scenario to what was described in the myth actually happened. Another reason why I doubt that the story is made up is because the Colchians actually did have contact with many other peoples including the Greeks. The wood of Colchis was known for its sturdiness and durability, fit for making high quality ships. Colchis, unlike the neighboring Iberian tribes (and later the Iberian state), was comparably mercantile and saw more profit in trade than in conquering lands. This is one of the reasons why the territory of Colchis remained unchanged for centuries. My point is that mercantile people like Western Kartvelians would probably have a fair amount of contact with Greeks, while Scythians would choose to raid or pirate the Greeks for profit. So just by thinking logically, if this story was indeed made up and Pelias' goal was to kill Jason by sending him to foreign and dangerous lands, wouldn't the Northern Black Sea coast be a more fitting destination? Instead of going to a partially centralised, mercantile region?