T'was cancelled.
tl;dr

wonder when the producers are gonna say why...

Well that's the producer(s) and creator's decision other than that, why not.

They were going to. But they scraped it due to less episodes being produced so they had to hurry. But they mixed the movie in. When Billy's Bucket List aired. That was supposed to vlbe the start of the movie. And supposed to end when they fix the problem with his dad.