I don't think it was clear Walker landed on the base on the Baez play--it looked like he probably barely touched the edge of the base, but the angles they had weren't clear.
It would've been an interesting challenge because it looked like the umpire signaled that Javy was out on the tag, not because Walker landed on the base. If the umpire's call was that Javy was tagged out and the Cubs challenged the tag, then I think they would've won. But then the Diamondbacks could challenge that the foot was actually on the base...and I think that might have been a "call stands" situation where there isn't enough evidence to overturn.