What were the odds of April and Casey hooking up up in this series?
 

I mean... All things considered?
...

To have that very same episode as an example...
https://vignette.wikia.nocookie.net/...20161013033320
"Looks like it's gonna be just you and me, Red!"
April giving a "And you'd better keep your hands to yourself, Buster!" look in reply.

Also in later season three episodes in Space, they are seen guarding the ship while the turtles and Fugitoid head out in Dimension X to talk with the Utrom council, Donnie makes no worried noises about them staying behind. While April and Casey are more peeved off about the fact that they're staying behind with only each other as company.

So where did it all go wrong?

Yeah, while in her body, Casey started touching April without her consent.

The odds weren't high since this show was never gonna explore serious relationships.

What happened with their relationship in this series? It seemed like they were staring one, and it's like it was just dropped from the story with no explanation. Or am I just not remembering them addressing not going forward with it.

I think it pretty much stopped after that 'Bigfoot episode' that showed that April actually didn't mind Donatello's affections. I guess once those finally kissed, it was time to drop the overall subplot. Maybe Casey would still try to hit on her from time to time but the overall love triangle stuff was dropped.