Well for a lot of reasons. This question reminds me of myself and my old high school mentality. I would ask myself, "If C++ has one of the fastest execution times, why would any company use anything else? Anything else (like VB, COBOL, Delphi) would be slower." This was bad thinking. But it was ok though. I didn't know much about real world programming or the business world.
If I'm your boss and I tell you to make an application that generates invoices, and be meticulous on the GUI portion, what would you use? Oh yeah, and I told you that I need it tomorrow morning. Would you use Assembly? Visual C++? Visual Basic? I think we know the answer to that. Rapid software development matters. Execution time is not always an issue. It does matter, and it should, but creating the whole application in Visual C++ would not be worth it for just gaining some performance time. Well, not for this specific application. You could if you wanted create libraries in C++ that would handle any load processing from Visual Basic. But you get the idea.
Also, what if you were under a Unix/Linux OS? Could you use VB? Nope. You'd be limited to the tools available under Unix/Linux (which are a lot by the way).
The same issue applies to web languages. Rapid software development could be an issue. In this case we could use ColdFusion. But ColdFusion costs money... so then the current budget would be an issue. We would need to go with something that was free or low cost. We could go with something like PHP, Perl or J2EE. But what if my staff is used to using Visual Basic and Office products? Do you think learning ASP/ASP.NET (which emphasizes strongly on VBScript) would be easier compared to ColdFusion, PHP, Perl, or even Java? This is a tough question. Just because it runs the fastest doesn't always mean it's the best.
Still there are debates on which runs the fastest (mod_perl, PHP with an accelator like Zend's, compiled Java/ASP.NET, or ColdFusion (which now compiles to Java byte code). Wherever you go, they claim that they're the fastest. If you go to a PHP web site, they claim it's the fastest, if you go to a Perl web site, they claim it's the fastest, and so on.
So before you go and select a server side language or application server, you should consider the following:
1. Company Budget (always be concerned about how much you'll save and how much you'll make). Be aware of open source solutions and how much money it will save your company.
2. What technologies you have already. What OS? What database server? How many computers and what types?
3. How easy will it be to pickup the language? If your staff already knows Visual Basic, why not ASP/ASP.NET? Why Java? If Java, what are the benefits?
4. When does the web site go live? How long will it take to develop? Does everything have to be built from scratch?
5. Does the technology you're choosing come with all the tools you need to create your application? For example, if you want to upload in ASP, you'd have to buy or find a free a component to add this functionality. Coldfusion comes with this. Java and ASP.NET already come bundled with a lot of functionality.
6. How easy will it be to extend/maintain/debug the application? To update? Will the syntax and facilities within the technology make it easier? How well will it integrate with the current technologies you have?
7. How does the application server (what runs ColdFusion, ASP, PHP, etc.) handle under load? Is it scalable?
8. How easy is it to get support? Is there a strong community?
9. If you go with technology you pay for, you have someone to point the finger at if something goes wrong. With open source software you have little liability. You don't have one single company you can sue that owns it. If you pay for a service, you're paying for a reason; to get a guarantee that it will work (even though this is not always the case).
10. Does the technology have a future? You don't want to spend money buying a technology and find out it will be discontinued soon.
11. How does the technology handle security? Internationalization? How does it handle external services (HTTPS, web services, LDAP) and other protocols?
Those are a few issues to consider. As you can see, you always start any answer to a question with "it depends." Even if someone asks you "what's 1 + 1?" You say "it depends, is it in binary?"
Hope this helped!