Because it's dominant. In other words, once a platform becomes dominant, it tends to stay that way. It's inertia. With millions of applications available for Windows, it makes sense to use Windows. And with everyone using Windows, it makes sense to write new applications for Windows. It's self-perpetuating. The greater the number of Windows computers in the world, the greater the inertia. People think that Microsoft creates this dominance, but that's not true. It just is. Even Microsoft can't influence it much. That's why there are so many people still on Windows XP and Windows 7. And even when this happens, businesses usually tend to get lazy with their products, not keeping them the best working