There are several reasons. I won't describe them all, some are just
vague, and not worth discussing.
One major con against perlmonks is its user interface. It's web based,
which is just horrible compared to usenet, mailinglist, IRC or MUDs,
all of which I can do from an xterm. For usenet and mailinglist, where
you write articles, just like with perlmonks, you can do the editing in
your favourite editor *IN PLAIN TEXT*. With perlmonks, you have to use a
small textarea, writing in some poorly defined HTML derived markup. You
don't even have the option to write in plain text, or POD. It's a lot
easier to type C<$_> than it is to do <code>$_</code>
(specially considering that HTML already has a CODE element). Mailinglist
and usenet articles I can automatically file - not so with Perlmonks.
Perlmonks is the only community I frequently visit that has thrown adds
at me. I very seldomly see spam in the usenet groups I visit, or on the
mailinglists - they have been filtered out before reaching me.
Also, I find it much harder to 'notice' individuals on Perlmonks. On
other forums, I find it easier to notice others - people spring out
because of their particular knowledge, style, behaviour, signature,
(dis-)likes, etc. There are a few names here I pay attention too, but
that's usually because I know the names from other elsewhere.
Note that I'm not saying I don't like Perlmonks, I wouldn't be here
if that was true. But if I have to rank the communities I visit,
Perlmonks would end last.