I don't think it's necessarily a bigger success. Maybe TV is a more forgiving environment than the box office? A low rated TV show can last for years and years, never cracking even the top 30 in the ratings. A $200 million dollar movie needs to make a crapload of money for the studio to consider making another expensive movie. Most Superman-related shows didn't have particularly large budgets and would stretch less money than it would take to make a single movie over 22 episodes. Most likely, one season of Smallville was less than half the budget of Superman Returns.