American company. Less extreme than Japanese tastes, but still there.
Big exclusive IP's are better suited to the US market. A first person shooter; a third person shooter; a WRPG; a racing sim & a dancing game (although, have those stopped now? Idk).
Compared to Sony whose biggest IP's are a racing sim; a third person shooter; a hack 'n' slash and, to a lesser extent, an FPS and a 2D platformer.
No reason to switch. USA was PS2-dominated; many users switched to 360 when Sony effed up so monumentally with the PS3. Microsoft messed up with the Xbox One but corrected many of their mistakes before launch; thus not really giving much incentive for people to switch allegiances.
Many people happy with their 360's and feel no need to switch to PS3 too as it gets all of the multiplats which is what the majority of people will play.
Because Microsoft understand the importance and focus a huge amount of resources there; while Sony spread their focus a little more evenly across the world.
Pick any of the above.