I've done this- I found the PPC a few years ago, introduced myself maybe a year ago, but real life gets in the way and I tend to vanish for months and then come and read the board and a bunch of missions both old and new. I do love the PPC. I'm also shy about contributing to discussions (though I enjoy reading them). So I don't know if other disappearing newbies stay to lurk, but that's what I've done!
(And sorry I replied a bit late; I lurk here somewhat intermittently.)

Don't count'em completely gone, though. They may be still around here.. Lurking in the shadows, waiting for a topic that interests them, and then punce on it...

In fact, I have been here for over a year and I could bet that a good chunk of the newbies don't even know who I am, because how little I comment.

The community here does seem a bit insular, with a rather high knowledge barrier to entry. Even reading The Original Series (which I have done) is quite a task. Using the average of the first, thirteenth, and last missions, (Chosen because first, middle, and last seems representative. The missions have words counts of 3570, 8119, and 7285, including the title and all author's notes.), I estimated the length of the original series at approximately 164441 words. For reference, that's about 2400 words longer than the first two Harry Potter books combined, about 4400 words shorter than the sixth Harry Potter book, around 8300 words longer the The Two Towers, and approximately 2.5 times the median length of a novel listed for sale on Amazon (which is 64000 words). The PPC canon has also been expanded upon significantly, and to a newbie it can sometimes feel like there's a requirement to be intimately familiar with every notable agent ever, which, even if it isn't an actual requirement, can still be intimidating.

Beyond that, it wouldn't be too difficult to see someone feeling that their writing, while probably sufficient, isn't good enough if they compare it against the veterans. I might be dealing with this (it could be that my writing just isn't good enough), as I only started writing creatively late in high school, and while my writing has improved from the impossibly dense stuff I created in high school, where I would compress several sentences into just a few highly ambiguous words that could be interpreted to mean everything I was trying to say (while I received praise for it sounding very nice, nobody could figure out what was going on). On top of that, some people might not be interested in/able to effectively write the PPC's fluffy comedy, which I know fits me at least for now, as I'm too used to writing very dark stories. Had I not read that you can be a part of the community without writing missions, I probably would have left.

Mainly due to a lack of material that I actually feel inclined to join in on. I'd much rather figure put how to start a conversation than jump in on one.

(the one at the bottom of every message)?

That, and it's all too easy for those updates to get sorted as spam.

Then there's always that pesky thing known as "Real Life."

;P

It just seems to happen, I guess. :P