During Season 6, Topher Grace (Eric Forman) and Ashton Kutcher (Micheal Kelso) wanted to leave the series, but decided to stay on for season 7, which was supposed to be the final season. But Fox decided to go on and make Season 8 without either cast member. The characters of Eric and Kelso were written out of the show. Fox brought in a new character named Randy to take their place. With two of the most popular characters gone, That 70s Shows ratings heavily dropped and Fox was forced to end the show. But Topher Grace and Ashton Kutcher still decided to return for the final episode of the series. I heard that Topher was not at the final goodbye episode because he was shooting spiderman 3, and i also read he did not get along very well with the other cast members anyway.