Nickelodeon has a bunch of shows of yesteryear (I love that word).
I have enjoyed many different shows that use to or do appear early in the morning, and I always feel bad when that show isn't running anymore.
Nick brought back Legends because it is Legends. Legends always seem to have a consistent record of being a popular show. They figured, instead of playing ChalkZone or Rocket Power, they could bring back one of the more successful shows on Nick.
Logically, they went backwards on Legends. The first new Legends on Nick was the Encrusted Egg. By playing the shows in reverse production order, they could stop whenever they wanted to. If they started with Galileo's cannonball, and made it all the way to the end of a season, they might have to keep going due to fans, or they may have to stop, despite of fans. By going backwards, they made all the calls, and the fans couldn't really have any control.
Why only 17 episodes? Not a clue. Maybe they thought not enough people were watching for them to keep the show running on a profit per se. Nickelodeon, as we know, have always been questionable about keeping shows on air.
We can only hope that now that Nick is making new versions of old TV game shows, then maybe...