No, it wasn't time and budget constraints. It was Spielberg being "creative" and not wanting to do a cliché explosion ending. He wanted something more... "poetic." However... when the network honchos saw the finished product they were incensed! They felt that Spielberg "owed" them a flashy, made-for-TV explosive ending, and they felt they hadn't gotten the product they paid for. So the network insisted the ending be re-shot to include the expected explosive ending, whether Spielberg agreed or not. After all, at the time Spielberg was a 24 year old kid, without the clout and "name" he has today. He was just a "hired hand" as far as the TV network was concerned. But the movie's executive producer had Spielberg's back, and refused to authorize a re-shoot. Ultimately, the network gave in and the movie ran as Spielberg intended it.