Passion drives an entrepreneur because entrepreneurship is something that is not at all easy. Entrepreneurship typically involves special vision and very hard work, a fair amount of stress, and very uncertain returns.
An entrepreneur is someone who is willing to take on risks in order to innovate. They are trying to start new businesses which, they hope, will allow them to make money. Because they are starting new businesses, and because they are often trying to do something that has not been done before, they are taking on risk. They are typically using their own money to try to make their business grow.
Entrepreneurs need passion to realize their vision and because they are not going to have an easy time. If they leave a job that they had been holding to open their business, they are likely trading security and relative ease for more work and less certainty. They will often have to work long hours. Even before they can do so, they must spend a lot of time making plans for their new business. They will have the stress of wondering whether their business will succeed and what they need to do to make that possible. All of this is much harder in many ways than working in an established company. Therefore, entrepreneurs need to be driven by passion (the passion for their new idea and/or the passion to be their own boss) so that they will want to take on all the problems that go with entrepreneurship.