Because addition of a single drug resistant trait is simple and the benefits are immediately observable. There are some experimental GM crop strains aimed at ac

Because addition of a single drug resistant trait is simple and the benefits are immediately observable. There are some experimental GM crop strains aimed at achieving better nutrient levels (golden rice, high lysine maize) or even better, improved photosynthetic efficiency, but these are much harder to commericalise for the lack of a single marketable "gimmick"