You're right to notice that the story of the thief on the cross who asks Jesus to remember him and is promised a place in paradise is unique to the Gospel of Luke. There are a few reasons why this might be the case:
* Luke's Emphasis on Mercy and Forgiveness: Luke's Gospel is particularly known for its themes of mercy, forgiveness, and inclusion of marginalized people. The story of the repentant thief fits perfectly into this overarching narrative. It highlights Jesus's willingness to offer salvation even in the face of death and to those considered to be outside of respectable society.
* Different Authorial Focus and Selection: Each Gospel writer had a specific audience and a particular theological point they wanted to emphasize. They would have selected and presented events in Jesus's life that supported these goals. The other Gospels might not have included the story because it didn't fit as neatly into their narrative emphasis.
* Source Material: It's possible that Luke had access to a source (oral or written) that contained this story, while the other Gospel writers did not. Gospel writers compiled their accounts from various sources, and the availability of specific traditions could explain differences in their narratives.
* Theological Message: The "paradise" aspect may also have been a particular theological emphasis of Luke. The idea of immediate reward and entry into God's presence after death may have resonated with Luke's audience, or it may have been a point he wanted to emphasize.
* Completeness of the Account: Some scholars argue that the absence of the story in the other Gospels doesn't mean it didn't happen. The Gospels are selective accounts, not exhaustive records of every event.
It's important to note that the story does not contradict anything in the other Gospels, and it aligns with the broader Christian understanding of salvation through faith in Jesus. The lack of corroboration doesn't necessarily invalidate the account, especially considering Luke's known emphasis on mercy and forgiveness.