Here are my views on the outside but who is peering through the windows. I just got my Third Class medical and am now looking at flight schools to earn a private pilot license.
1. Lack of information / commonality. Want to buy a gun? There are 67k or 69k gun dealers in the USA; finding a shop even in the most restrictive states is relatively easy. Want to buy NFA/Class III? Google and (until recently) the corporate or trust methods got around the most paranoid CLEOs. Language/lingo a problem? It's relatively easy to get things deciphered. Want to learn about aviation? Honestly, if it wasn't for my Grandfather building three of his own planes, being lucky enough to know some private pilots, and having the EAA only 1.5 hours from here, I'd be lost. All airports have the security mystique around them, even if it's a truly friendly GA airport. Books? Google? Yeah, there's info-- but it's easier to find good info in plain english about other stuff. Aviation is far more complex and technical than most pursuits.
2. Cost. Anybody can get into shooting for $500.00. That's a 10/22, a .22 pistol, and a brick of .22. (Maybe we'd have to go with used firearms given the current ammo price shocks.) $500.00 to get into flying?
Okay, how about $5,000.00?
Flying may not be for everybody (the FSA who refuse to accept responsibility for their actions become a lot deadlier in the air, those with truly serious health problems/seizures, etc.), but right now it's limited to folks who have $10,000.00+ to burn just to get their license. That's not counting anything post-license; the cost to rent a plane, avgas, insurance, the cost to buy a plane, hangar rental....... Aviation is priced outside of the means of most folks. Yes, there are ways to cut costs, but it's still not a cheap hobby and we are in a poor economy.
Both of these issues, plus our economy, contribute to the decline of GA. Fewer GA participants mean that there will be fewer people around to object when the government engages in negative activities toward GA. Then we have a third thing contributing to the decline of GA- unfavorable legislative requirements. You know, sort of like folks trying for firearms ownership in some areas. And more people driven out means less people in GA which means less people to raise hell when whoever proposes some additional legislative burden... And the cycle continues.
Mike