Currently, I have no source citation, but I'm pretty sure, that the series was planned for a running time of 65 episodes in five seasons.
After that didn't work as expected, the creators decided to push the series into a new direction with season 3. That's, why the last season is so different from the first two. After the third season also ran fruitless, the creators changed the direction after half of the last season and put in supernatural powers, which they used to finish the series.
I think, it's also posted somewhere else here in Fiver's Honeycomb, but I can't find the original topic.