Explanation for Frank going back to being a slob in the first episode...
- It's an Alternate Universe in which Shane was never born. Simply. Considering that, in the pilot episode, Ozzy mentions being in "Snot Patrol" when he should have been promoted to something much higher after the events of the movie, this makes the most sense.
- Or it happened after Ozzy's award ceremony and before his hike with Shane.
- Shane died in an accident during the hike and Frank reverted back to his old habits as a coping mechanism.
- Borrowed from the movie's Fridge Horror entries: He was unable to maintain any attempts to become healthy because of the brain damage incurred from his massive fever—104 degrees Fahrenheit, while not guaranteed to be lethal, is the threshold for permanent brain damage.
- Or Shane grew up and went off to college, and, either out of loneliness or for lack of anyone to hold him accountable(which, if we combine this with the above fridge horror, would be a necessity for Frank to make any progress), Frank reverted back to his old habits.
Ozzy and Drix would have eventually returned to Frank.And Maria, Dander, and Backseat would have come with them.