There are issues with competing implementations, but I would say that's not totally bad(options are good) and over time, some equals become more equals than others. Python has numerous web frameworks, attributed to the fact that it's easy to define your own, but a newbie is more likely going to stick with Django.
As I said, people re-implementing things "just because" isn't unique to lisp and might not be as bad. Look at Flask - it was an April Fool's joke by Armin which is now a proper micro framework. It isn't like Django wasn't the dominant and recommended framework when Flask came into being. It's good to have options and progress depends on people fooling around.
Look at async scene. You have twisted and you have gevent and you have diesel. Templates? Jinja2, Mako, Django, Cheetah etc.
Python or Lisp, most of the people are going to make their choices and stick with it. It's not like everyone who programs lisp starts writing their own CLOS, and not everyone who programs Python tries to write his own framework regardless of how easy it is.
> For obvious reasons, that’s not the case with Lisp dialects despite their superficial similarities, but I think that catches many neophytes off guard.
I don't know. Doesn't most of the programmers program to a particular scheme or lisp implementation(sbcl, racket, clozure, gambit, clojure)?
> So even if such detrimental personality traits aren’t peculiar to Lispers, there is a strong apparent correlation because of the “fractured” community.
I think more than the "fractured" community, it depends on the out of the box experience. If I am programming Racket, I won't try to build an object system of my own - the one it provides is good. If I am programming Clojure, though it doesn't provide a conventional object system, I agree with the choices and won't try to implement my own.
> I don't know. Doesn't most of the programmers program to a particular scheme or lisp implementation(sbcl, racket, clozure, gambit, clojure)?
Common Lisp libraries seem to be getting much more portable across implementations, so within the CL dialect the language implementation is becoming less of an issue I think. And QuickLisp is one counter point to the OP -- it has broad community support.