Eric at LispCast wrote an article about why PHP is so ridiculously dominant as a web language, when arguably more powerful languages like Common Lisp linger in obscurity.
I think the answer is pretty easy. In real life, practicality usually trumps everything else. Most programmers aren't paid to revolutionize the world of computer science. Most programmers are code monkeys, or to put it more nicely, they're craftsmen who build things that other people pay them to create. The code is a tool to help people do a job. The code is not an end in itself.
In real life, here's a typical situation. You have to make a website for your employer that collects survey data from various people out in the world, in a way that no current off-the-shelf program quite does correctly. If you could buy a program to do it that'd be ideal, but you can't find a good one, so you decide to write one from scratch. The data collection is time-sensitive and absolutely must start by X date. The interface is a web page, and people are going to pointy-clicky their way through, and type some numbers, that's it; the backend just doesn't matter. For your server, someone dug an old dusty desktop machine out of a closet and threw Linux on there for you and gave you an SSH account. Oh right, and this project isn't your only job. It's one of many things you're trying to juggle in a 40-hour work week.
One option is to write it in Common Lisp. You can start by going on a quest for a web server. Don't even think about mod_lisp, would be my advice, based on past experience. Hunchentoot is good, or you can pay a fortune for one of the commercial Lisps. If you want you could also look for a web framework; there are many to choose from, each more esoteric, poorly documented and nearly impossible to install than the last. Then you get to hunt for a Lisp implementation that actually runs those frameworks. Then you get to try to install it and all of your libraries on your Linux server, and on the Windows desktop machine you have to use as a workstation. Good luck.
Once you manage to get Emacs and SLIME going (I'm assuming you already know Emacs intimately, because if you don't, you already lose) you get to start writing your app. Collecting data and moving it around and putting it into a database and exporting it to various statistics packages is common, so you'd do well to start looking for some libraries to help you out with such things. In the Common Lisp world you're likely not to find what you need, or if you're lucky, you'll find what you need in the form of undocumented abandonware. So you can just fix or write those libraries yourself, because Lisp makes writing libraries from scratch easy! Not as easy as downloading one that's already been written and debugged and matured, but anyways. Then you can also roll your own method of deploying your app to your server and keeping it running 24/7, which isn't quite so easy. If you like, you can try explaining your hand-rolled system to the team of sysadmins in another department who keep your server machine running.
Don't bet on anyone in your office being able to help you with writing code, because no one knows Lisp. Might not want to mention to your boss that if you're run over by a bus tomorrow, it's going to be impossible to hire someone to replace you, because no one will be able to read what you wrote. When your boss asks why it's taking you so long, you can mention that the YAML parser you had to write from scratch to interact with a bunch of legacy stuff is super cool and a lovely piece of Lisp code, even if it did take you a week to write and debug given your other workload.
Be sure to wave to your deadline as it goes whooshing by. If you're a genius, maybe you managed to do all of the above and still had time to roll out a 5-layer-deep Domain Specific Language to solve all of your problems so well it brings tears to your eye. But most of us aren't geniuses, especially on a tight deadline.
Another option is to use PHP. Apache is everywhere. MySQL is one simple apt-get away. PHP works with no effort. You can download a single-click-install WAMP stack nowadays. PHP libraries for everything are everywhere and free and mature because thousands of people already use them. The PHP official documentation is ridiculously thorough, with community participation at the bottom of every page. Google any question you can imagine and you come up with a million answers because the community is huge. Or walk down the hall and ask anyone who's ever done web programming.
The language is stupid, but stupid means easy to learn. You can learn PHP in a day or two if you're familiar with any other language. You can write PHP code in any editor or environment you want. Emacs? Vim? Notepad? nano? Who cares? Whatever floats your boat. Being a stupid language also means that everyone knows it. If you jump ship, your boss can throw together a "PHP coder wanted" ad and replace you in short order.
And what do you lose? You have to use a butt-ugly horrid language, but the price you pay in headaches and swallowed bile is more than offset by the practical gains. PHP is overly verbose and terribly inconsistent and lacks powerful methods of abstraction and proper closures and easy-to-use meta-programming goodness and Lisp-macro syntactic wonders; in that sense it's not a very powerful language. Your web framework in PHP probably isn't continuation-based, it probably doesn't compile your s-expression HTML tree into assembler code before rendering it.
But PHP is probably the most powerful language around for many jobs if you judge by the one and only measure that counts for many people: wall clock time from "Here, do this" to "Yay, I'm done, it's not the prettiest thing in the world but it works".
The above situation was one I experienced at work, and I did choose PHP right from the start, and I did get it done quickly, and it was apparently not too bad because everyone likes the website. No one witnessed the pain of writing all that PHP code, but that pain doesn't matter to anyone but the code monkey.
If I had to do it over again I might pick Ruby, but certainly never Lisp. I hate PHP more than almost anything (maybe with the exception of Java) but I still use it when it's called for. An old rusty wobbly-headed crooked-handled hammer is the best tool for the job if it's right next to you and you only need to pound in a couple of nails.