There's also one from 2008
Which is significantly different from the first two.
Because it's a relatively cheap and stable walker. AT-ST and other bipedial walker's are inherently unstable, because it would have to constantly readjust its self to stay upright. Which obviously bricks can't do, and no kid want's to play with a walker that can't stand up on it's own. They could bulk up the legs, but that would take away form the look of the set. Larger quadrupedal walkers, like the AT-AT (110$) or Hexapedeial walkers like the AT-TE are more stable, but more parts intensive, making it hard to sell at a somewhat lower price point (90$). a three legged walker like the AT-AP Is stable, and is less parts intensive then the bigger ones. At least, that's how I interpret it.