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Because people are freaked out about anything abnormal.
GM was messing around with ultra lean burn engines that used a NOx sponge (a la VW 2.0L TDI) that required a NOx regen once in a while. Apparently ammonia, created by briefly running rich through a catalyst (you know when you smell ammonia behind a car going up a long hill?) goes into the NOx sponge and converts the NOx into happier gasses once the NOx sponge is full. No DEF (water/urea) required.
Remember that DEF costs money too, so the increased fuel mileage must offset the added cost. Diesels typically burn 3-5% DEF in relation to diesel fuel, and DEF costs about $4/gallon.
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