Why is the bedrock buried so deeply in much of northern Minnesota?
As njuffa mentioned I don't believe you are looking at soil 100s of meters deep you are looking at unconsolidated sediments 100s of meters deep.  

Depending on the geological process it is possible to deposit 100s of meters of sediments in a very short time.  The conditions for deposition of large amounts of sediments are frequently found during the period of time that the glaciers are melting.  During this time the sides of the glacier end up forming the glacial fluvial terraces and the toe of the glacier often breaks into pieces of ice that the sediment can build around.  

This site from the Minnesota State University describes the majority of the lakes in Minnesota as Kettle Lakes, which suggests that the tail end of the ice age was a time of significant sediment deposition.  The North Eastern area would have likely be covered by ice while the South Western portions would have been experiencing the deposition of materials from the rapidly melting glaciers.  There is a whole wikipedia site devoted to the glaciation of Minnesota that describes the events relatively well.