The downgrade is because the US Gov is spending far more than it's taking in. In other words, it's for the same reason that every other person / business / government gets downgraded. It's important to realize that there's no magic here.
The reason the European debt crises is so serious is because the adoption of the euro means that no individual contry can print their own money to avoid default.
It's more because each individual country issues bonds. The ECB can't issue debt.