The fact that it’s the least well known of the series makes it the best choice for the first remake of the franchise.
For the most part, people tend to skip over it and go straight from Sean Connery into Roger Moore.
The original Casino Royale isn’t considered part of the same series and Never Say Never Again, which was a remake of Thunderball, was also a separate project.
The decision of what that second movie could be is also an interesting one.