Why Lisp Is Unpopular
A picture is worth 1K - 3 * 2^3 words:

http://www.hq.usace.army.mil/cepa/pubs/jul00/radar.jpg

That's a phased-array radar installation that (I understand) was built to detect Soviet ballistic missiles inbound over the North Pole. Google has more images.

The term "Phased Array" has interesting implications here. It means that received phase information is preserved among the many small antenna elements on each face of the structure. That only works if the paths traversed by the antenna outputs are all the same length (modulo lambda), with a margin of error around .1 * lambda, where lambda is a representative wavelength of the received signal.

This thing uses microwave signals [1], which means we can reformulate the preceding paragraph as follows: You're looking at a picture of a single circuit the size of a large office building, built to a precision of 100 microns. Now's a good time to point out that little yellow thing in the lower left corner of the image is probably some kind of earth-moving machine.

The US military built a number of these installations in Alaska and Canada during the Cold War. Let's pause here to think about all the structure-hardening and weatherproofing that must have been involved.

What does this have to do with Lisp? It's not an exaggeration to say that building and maintaining these early warning systems required the attention of at least a third of all American microwave engineers [2]. When the Cold War ended, most of them left the big contractors the DoD hired to build the radar and started doing basically the same work for cellular companies [3]. Ten or fifteen years later, mobile phones became ubiquitous.

In the case of Lisp, there was no continuity in the transition. Common Lisp, in particular, was primarily in use by a rarefied group of specialists working on room-sized computers at places like DARPA, thinking about things like AI for driving tanks across central Germany. When the funding dried up and the specialists had to move on [4], they found work writing C, Perl, or Java on microcomputers. So Lisp lost its user base and the last of its major hardware platforms (now that you couldn't buy a Lisp Machine any more) all at once.

Of course, when programmers discuss Lisp's continuing lack of popularity, regardless of their opinion of the language itself, they seem a lot more willing to blame things programmers ultimately control, like the social habits of Lisp users or the damn parentheses.

[1] I'm not a microwave guy, so some of this explanation is simplistic almost to the point of inaccuracy. In particular, I don't know what frequency bands these installations use.

This is as good a place as any to point out that I also have no special knowledge about Lisp at big organizations after the end of the Cold War. People who do are strongly encouraged to email me if I've gotten anything wrong.

[2] I don't have a citation for this number, but one of my old professors does. I'll see if I can get him to email it to me.

[3] "3G" cellular technology would be unworkable without big antenna arrays

[4] A lot of the "classic" books on Lisp and related topics were written during this period. SICP, PAIP, and On Lisp, and The Seasoned Schemer come to mind.