Crops produce the greatest
amount of waste, mainly from
plant parts not harvested as
food. why are most people less
concerned about crop waste
than other types of waste?

Because crop 'waste' is waste only because you can't sell it or eat it. 
Unlike other forms of waste, it's not toxic and it's not a pollutant.  The
'wasted' part of the crop is intentionally plowed back into the ground in
many cases.  It returns nutrients to the soil that were extracted to grow the
crop, and thus serves to partially replenish the soil for the next year's crop.