Because Shimano, who knows better then us mere mortals about what gearing we need, decided after 10sp. to stop making triples in the upper end - 105, Ultegra, DA. Deciding instead that SRAM had the right idea and put all 11 or 12 gears in the back.
They do make a triple for the next level down - Tiagra, which a pretty good group, so all is not lost.