The Internet changes rapidly. If you are too locked into one idea, you may not adjust to the market fast enough. If you hang on too long, you may be the proverbial captain who goes down with the ship. A good manager who holds no emotional investment in the idea may be able to see more clearly. The opposite can also be true.
Many visionary people jump from one idea to the next.
They may not stay focused long enough to guide a project to completion. I've seen this type of manager continually change projects mid-stream until, hundreds of thousands of dollars later, they still have nothing substantial. This type of people needs a solid vision to guide the project through to completion. People who have big ideas often have big egos. They want to control every piece of the project.