It was SO short-sighted of Paramount to think that a movie couldn't be good promotion for a current TV show- something Fox proved later on with The X-Files.
Economics. "X-Files" had a main cast of two characters and was set in the modern day. Signing your main cast to contract periods longer than 5+1+1 years is extremely tricky
. Series like "Friends", "Cheers" and "Frazier" had large casts and went beyond seven years, but they were only half-hour eps and, again, were set in contemporary times.