Hmmm. Probably because it is a Gambit solo story and Legends is just the name of the book.
So what if it was a story in the past revealing that Gambit was really working for Sinister with a mission to destroy X-Men from within and would betray and fight Sinister and refuse to work with him at the end?
I would make it with time jumps and adding some canon existing moments that changed his motives. So the final battle would be after X-Men #45 with emotional impact on Gambit from a story with Rogue and Sinister cameo that soon Gambit must betray the X-Men.
It can be done as a solo Gambit story without X-Men even finding out at the end about his bad motives before he chooses a side of good. Or maybe only Storm would know and keep it as his secret because he chose the right choice at the end.
I would love that