That was thanks to the bump they got from the 1982 pennant-winning team. But after 1983, they were below the AL average in attendance. They drew a decent amount of people in 1987-89, but they won 91 games in '87 and finished only 2 games out in '88.
Their attendance has been much better in Miller Park, but it really took in 2007. That year was their 1st playoff contender in ages and they only finished 2 games back. After that, they made the playoffs in '08 and '11. So, success on the field has played a big part in their attendance boost.