Because Nintendo software has been habitually been good, allowing them to weather flops, whereas a lot of Segas stuff has been shit, meaning that their relatively far fewer quality hits didnt keep the hardware boat afloat.
Plus, to quote a very bad person, quantity has a quality all of its own. Nintendo sales have essentially always been greater than Segas, so comparatively the hits have brought in more money and the flops have hemorrhaged less money.
Oh, and Nintendo has a tendency to better stick to budgets and to market their hardware and software to maximize, or at least guarantee, profits, whereas Sega has been forced to be desperate and oftentimes dug holes they couldnt climb out of.
Irregardless, for all intensive purposes, I could care less.