1) Groupon taks a large cut on top of the big discount. It used to be that GroupOn wanted a 50% discount and then 50% of the revenue. This is like running a 75% off sale.
2) The customers are poorly-qualified leads. All we know is that they are local and like a good deal. That doesn't give the retailer much indication that they would be a good customer (and I think we have seen that they aren't).