Is Python good enough for big applications?
I'm going to answer your revised question about why would anyone use a different language or technology stack.  First, I love Python.  Great language and definitely has its merits.  However, I choose C#/.NET as my technology of choice and use Python for scripting.
Yes, Python as a language is very simple/clean and there isn't compile time.  However, I find it many times easier to work with a strongly/statically typed language.  #1 reason is IntelliSense (and I mean GOOD IntelliSense - haven't been satisfied with Python editors in this regard).  IntelliSense makes a huge difference in ease/speed of development.
C# is backed by Microsoft (for better or worse).  While the h8t3rs out there can throw rocks, it is hard to deny that Microsoft has technology for the entire stack (desktop, server, web, mobile, etc.) and it all integrates very nicely.  I also know that it is supported by hundreds of developers who are dedicated to providing the best experience possible.  I also can go to a plethora of sites and watch videos, read articles, and find what I need for just about anything.  Support is nice.  
I know Python has an excellent (and large) range of libraries and frameworks at its disposal, but I have also felt a certain lack of continuity between the various projects.  Use this 3rd-party library here, throw this 3rd party library in there, use this open source project here...And while I love open source, it takes a much longer time to reach the quality in which Microsoft has put out.
Lastly, Visual Studio is one of the most excellent IDE's I've ever experienced.  I know there are many fine text editors out there (I love you VIM!), but it is hard to beat the integration of source control, code editing, compiling, building, publishing, testing, and deploying all wrapped into a most excellent package.
All this to say, Python is great and can perform wonderfully on large sites - don't get me confused.  However, there are valid arguments for using enterprise technologies.  It just depends on what resources you (or your team) will need, what you are already familiar with, and how you plan to scale when your application is successful.  If you are comfortable with the technology and libraries, have a community to answer tough questions, and can compartmentalize your code so that if you ever do need to re-write it, it is already broken up, then I say you will do just fine.
From the moment I have faced Python, the only thing I can say for it is "It is awesome". I am using Django framework and I am amazed by how quick things happen and how developer friendly this language is. But from many sides I hear that Python is a scripting language, and very useful for small things, experiments etc.
So the question is can a big and heavy loaded application be built in Python (and django)? As I mainly focus on web development, examples of such applications could be Stack Overflow, Facebook, Amazon etc.

P.S. According to many of the answers maybe I have to rephrase the question. There are several big applications working with Python (the best example is You Tube) so it can handle them but why then it is not so popular for large projects as (for example) Java, C++ and .NET?