Sites are getting savvy on the distribution of coupon codes. Often, they are given regularly to members with certain numbers, and then, only for a limited time. Or, they are given for things like free shipping in conjunction with other promotions (for example, during sales times, to decrease price resistance.) You guys do realize that it was only a matter of time, right?
There is absolutely no incentive for a retailer to sell something for less than they have to. A running 20% coupon pretty much ensures suboptimal profits on high sell-through items, and doesn't do much to move items that are selling poorly. You might help your cashflow situation if you are having a hard time, but you end up with a worse problem at the end of the day. Having a sale on select goods, and then tacking on free shipping is a much better solution, from a retailer's point of view.