Wisconsin is mostly rural, so it's never had a huge population, but recent drops in population are a bit worrying. Despite being mostly rural, taxes are high compared to other rural states, and a lack of good-paying jobs has caused working-age adults and retirees to seek homes in other states. Wisconsin is also known for some pretty brutal winters, which just aren't worth enduring for a lot of people anymore.
That said, the state's total population has only fallen slightly in recent years, and it was experiencing growth before that, so it's possible things could turn around in the near future.