The banks (and their asset managers) put them on the market when they have clear title to the property. In Michigan a foreclosed upon home owner has a 6 month right of redemption (normally) to the home if they haven't abandoned it.
The banks and asset managers spend time after the notice of foreclosure & sheriffs sale to get their ducks in a row, so to speak. They are probably not ready yet to entertain offers on a recent foreclosure.
Keep your eyes peeled for the home to be re-listed. Work with an agent you trust to pull public record and they can give you a probable time-line for when it may be re-listed. Then, when it does become listed, bid aggressively. Banks today are often strategically underpricing homes and thereby getting multiple offers on the same home, driving prices up. Don't be surprised if it gets listed low and then sells for much higher if it's in good condition.