The correct question is:
"Why is there a NEED for govt to restrict carrying weapons in a national park?"
When restricting a right the burden of restriction is on the govt not the individual. For a constitutional restriction to be found unconstitutional the individual doesn't need to show a NEED the individual simply needs to show the govt hasn't met the requirements for restriction. If the burden was on the individual then burn the Bill of Rights because it is not worth the paper it is written for. The govt could simply set the bar for "need" so high as to make rights in name only. Not just the 2nd amendment but any right.
If govt restricts speech, religion, press or any right (which it does in limited circumstances) there must be an overwhelming need to do so.
That concept of the burden of restriction in the United State is called Strict Scrutiny.http://en.wikipedia.org/wiki/Strict_scrutiny
To pass strict scrutiny, the law or policy must satisfy three prongs:
First, it must be justified by a compelling governmental interest. While the Courts have never brightly defined how to determine if an interest is compelling, the concept generally refers to something necessary or crucial, as opposed to something merely preferred. Examples include national security, preserving the lives of multiple individuals, and not violating explicit constitutional protections.
Second, the law or policy must be narrowly tailored to achieve that goal or interest. If the government action encompasses too much (overbroad) or fails to address essential aspects of the compelling interest (under-inclusive), then the rule is not considered narrowly tailored.
Finally, the law or policy must be the least restrictive means for achieving that interest. More accurately, there cannot be a less restrictive way to effectively achieve the compelling government interest, but the test will not fail just because there is another method that is equally the least restrictive. Some legal scholars consider this 'least restrictive means' requirement part of being narrowly tailored, though the Court generally evaluates it as a separate prong.
Just because you don't like guns, or you don't see a reason isn't valid under strict scrutiny. Hell I don't really see a reason for blogs, some people might not see a reason for DU, others might not see a reason for fictional books (only book you need is the bible). There is a danger on banning stuff because you "don't see a need".
This is why the EXACT OPPOSITE is true. Read about Strict Scrutiny with an open mind and you might learn something.