Zeus is hardly even involved; Athena is the driving force of the prosecution each time, with Hera glomming on almost like she's currying favor.
Which is weird, but actually also the vital skeleton of an effective fanwank for the whole Olympian fail from start to finish. See, making Diana a god was Hera's idea. She did this without, as far as we know, consulting or even mentioning it to Athena. This is, at the very least, a breach of etiquette, which goes some way toward explaining why Hera is so willing to take a backseat to Athena through the whole ordeal. But what's done is done, right?
Except, Athena doesn't want Diana to be a god. Diana on Olympus is counterproductive - it's bad for Athena (and the Olympians in general) who's just lost an enormous amount of power and influence in the mortal world, and it's bad for Diana, who is fundamentally unsuited by character to the life of a deity. The whole thing is just a Bad Idea, really, and Athena is not generally in favor of Bad Ideas.
So she manipulates her family, in high Athena form, into booting Diana back down to Earth where she belongs. It's not a perfect solution, but it's much closer to ideal... and given the total random, arbitrary and really forced way Diana is stripped of her godhood when Luke starts up his run, I think it's quite possible to assume Athena's plan just wasn't quite done yet.