Philosophy-wise, I think there are two things that lead to C++ not having a garbage collector (which seems to be what you're getting at):
Compatibility with C. C++ tries to be very compatible with C, for better or worse. C didn't have garbage collection, so C++ doesn't, at least not by default. I guess you could sum this up as "historical reasons".
The "you only pay for what you use" philosophy. C++ tries to avoid imposing any overhead above C unless you explicitly ask for it. So you only pay the price of exceptions if you actually throw one, etc. There's an argument that garbage collection would impose a cost whenever an object is allocated on the heap so it couldn't be the default behavior in C++.
Note that there is actually quite a bit of debate about whether garbage collection is actually more or less efficient than manual memory management. The better garbage collectors generally want to be able to move stuff around though, and C++ has pointer arithmetic (again, inherited from C) that makes it very hard to make such a collector work with C++.
Here's Stroustrup's (not really direct) answer to "Why doesn't C++ have garbage collection?":
If you want automatic garbage collection, there are good commercial and public-domain garbage collectors for C++. For applications where garbage collection is suitable, C++ is an excellent garbage collected language with a performance that compares favorably with other garbage collected languages. See The C++ Programming Language (3rd Edition) for a discussion of automatic garbage collection in C++. See also, Hans-J. Boehm's site for C and C++ garbage collection.
Also, C++ supports programming techniques that allows memory management to be safe and implicit without a garbage collector.
C++0x offers a GC ABI.
