Because it's easier to conceal a 19 sized frame than a 17 sized frame. Have you not seen how many people cut down their 17 frames to fit 19 mags(or 19 frames to fit 26 mags)? I'd be willing to bet that more people would prefer a smaller frame and a longer slide than vice versa.
But Glock is a lot like Apple, they tell their customers what they want, not ask them what they want, and people still buy the hell out of them.
