At the end of the day, it boils down to this question...
Why would the government want to cover up the existence of Bigfoot?
What do they have to gain, one way or another?
I've read many theories as to why the government would do this...
Most are based on some kind of economic incentive, involving habitat and natural resources, while other theories focus on keeping the public in the dark in order to avoid mass hysteria or fear about using our national parks. Again, neither theory holds water, in light of other species, people and organizations that have or have had a very real and direct impact on natural resources and public morale. Why Bigfoot and not a rare species of owl? Why Bigfoot and not Exxon? Why Bigfoot and not one of the hundreds of serial killers we have read about in the news?
I just can't see the motivation to keep it out of public eye.
That being said, if the government really does have a policy ,to cover up the existence of Bigfoot, then this policy has been in effect for a VERY long time. This, more than anything, makes it very doubtful. A national cover up, spanning multiple decades, to keep the public from discovering a free ranging, native, North American animal, is very hard to swallow. Not to mention, the effort and financial resources needed would be very hard to cover up for any length of time.