I have noticed, that I have noticed similar things, other places. In this forum/site, JB and other authorities will notice certain traits and tendencies of new posters, patterns into which most seem to fall, probably because by this time, the site and its functioning is pretty well established and routine, so, by chance, there are only so many results that will likely be obtained from new members being introduced into the "environment".
Although some more active, veteran members also tend to be able to get this same handle on trends like this, these kinds of observations may bypass casual (even if long term) members or especially relatively new people. I think this is a case-by-case basis, per website, and the details and behavior are specific to each website, for the most part, so lengthy introductions at other sites may have entirely different dynamics and likelihood of continued participation, due to the rest of the site's features and functioning.
Here I can understand to an extent, as some will no doubt simply be curious people who have happened onto the site and/or felt the urge to make a pressing post, more like a single blog "core dump", and once done, they lose interest and forget about it, which is probably more a general tendency of that person, than anything having to do with ID.
Many simply have seen the site/forum as simply another one to join to see what it had to offer them, to entertain them or methods to allow them to entertain themselves, and finding little in keeping with their own goals, they leave, or their behavior itself is incompatible, which more often is the case with younger posters, with more immature or irreverent attitudes, but can easily include older posters who feel, in general, more freedom on the internet, to act in ways that are perhaps different than they do in normal life, and aren't able to appreciate the stability and level of consistent maturity maintained by the forum rules.
Others I have definitely seen become confused, frustrated and insulted, and suffer from information overload, being confronted with the very strongly emphasized requirement to make an individual introduction post, then familiarize themselves with what, to the average surfer, is an inordinately large number of rules, some of which probably seem unusual or even baffling. The apparent forum bots and corrections and editing and administration, each seeming to have their own particular styles and leniency and level of cordiality, strike many as out-of-place and extremely strange, with no particular explicit "authority" or equation of them as administrators or moderators, and for some, worst of all, are impersonal automatons which probably strike them like a "phone tree" where user input and replies are all automated.
Lastly, the forums themselves probably seem "busy" to some, with counterintuitive categorization and hierarchy, as well as different types of stickied threads and announcements and multiple sectioned threads, some of which are meant to also serve as the only categories in which to post, in some headings, while other headings freely allow the creation of new threads. The somewhat distracting odd graphic, coupled with the seeming inconsistent "last post" date and time for different forums, could be more and aesthetic turn-off than anything else.
Those are my guesses.