Well, we didn't...
Here's a sobering and prescient piece (which I saw referred to in comments to this Fire Dog Lake post wondering who would be Obama's John Mitchell (pointing out that the president can't decide someone's guilt), which I was sent to by this Glenn Greenwald article discussing Obama's doing just that in the case of Bradley Manning. Both of those are excellent posts, too.).
This one is by Arthur Schlesinger, written in May of 1974, and called "What If We Don't Impeach Him?" It's a warning about the Constitutional and moral dangers awaiting the country if Nixon was not impeached. Here are two snippets to whet your appetite (select to get more readable copies):