Bill Gates is one of the most successful entrepreneurs of all time because he happens to be one of the few people who have been able to capitalize on their own brilliance, innovation, vision, and shrewdness. At the same token, he is also someone who cares about the condition of humanity. Much of this can be seen through his maneuvering past competitors to achieve unimaginable success and his philanthropy.
In looking at his career and inventions, he not only secured ownership of one of the most successful operating systems by just understanding the importance of MS-DOS and how it might evolve to Windows only a few decades later.
His shrewdness got him past being sued by the government while at the same time his commitment to public service and humanistic principles can be seen through his philanthropic efforts.
Gates has been such a success because he never let success be a limitation, and when looking at all of his accomplishments (each one building on another), he always strived to make a better product, whether it was technology-related or through his philanthropy.