Declining 9/11 Truther activity is discussed in a thread topic here.
Dissimilarly, I notice
UFO/ET/alien visitation topics and speculations apparently remain popular with some.
What gives it legs?
A few of things:
With something like 9-11, it's a single event. It happens once, to a specific group of people and then it's over. It's analyzed and reanalyzed until there is so much evidence that the official story becomes impossible for the rational to deny, so what's left of the truther movement gets relegated to the extreme fringe even by other conspiracy theorists. At the same time, the debunkers get tired of engaging with them. How many times can you tell someone the sky is blue before you just give up and quit bothering?
Also, with something like 9-11, distance starts to prove things wrong. For instance, some truthers claimed that 9-11 was supposed to be a false flag so the government could take over, yet by and large 15 years later, the average American's life hasn't changed. The only real difference is longer wait times at the airport, which is hardly the 1984 scenario proposed by some truthers.
Additionally, distance also means lessening public interest in the debate, especially as the debate becomes more and more academic and less and less immediately relevant to our daily lives. How often do you see new thread on the JFK assassination? My guess is that many truthers simply begin to move onto other items as new issues arise in the media.
With UFO's, it's a very different animal. Many, many people have had "experiences," and many continue to have such. It has a much wider appeal, both because so many have witnessed things, and because, in a way, UFO's are much easier to believe in. The idea of life on other worlds is widely accepted as a real possibility by main stream science, not to mention pop culture, so it's really not a huge leap for the average Joe to imagine that aliens might be visiting Earth.
Also, aliens are cool. Most people think that discovering aliens would be awesome, so there is a desire for them to be real, at least on some level. Not so much with an evil shadow conspiracy to murder thousands of people. That added to continued pop culture interest in aliens and the thousands of encounters each year, it becomes easy to see why Ufology remains popular.
On top of that, it's impossible to disprove. Unlike the one event that can be investigated, with tons of evidence and eye witnesses, most encounters are difficult to entirely explain, often because they are vague or have little evidence to be evaluated. Not to mention that for every sighting that does get explained there are a thousand more no one ever even bothers to tackle.
I think this type of stuff keeps Ufology mainstream, meaning it attracts new followers, adds new "evidence" each year, and retains it's old followers much better than a truther movement does.