A corporation will use your information to make money, and they only have access to data that you freely provide them or their partners.
The government can gather your information covertly, can obtain data from multiple companies using legal means unavailable to corporations, and typically do so with the goal of charging one with a crime.
A corporation is composed of people, who are corruptible. There must be many people at Google (just to name one high-profile company) who have access to the email archive and search history of pretty much everyone they know. It seems inevitable that someone will misuse that information if it hasn't happened yet.
The key phrase here is "using legal means unavailable to corporations".
Governments retain a monopoly over things like use of force, forcible investigation into your affairs, etc. And when they abuse those powers, you legally have significantly less recourse than you would if a private company did the same thing to you.
