Fans watch the game for passion(to cheer their team to WIN) and entertainment. with attacking football they might get both, with defensive football, fans that have passion might still fill the stadium.
With a continuosly WINNING football team playing somewhat defensively. You lose fans that want entertainment and you lose many Passionate fans that want to cheer the team to victory due to the fact that we always win. So these fans lose some of the fun of supporting the club, a Juve win is too predictable.
Thats how I see it anyways.