Game producers surely are adapting quicker to the multi-core architecture than the 64 bit architecture. I think that is what Puki meant. Obvious, because you don't have to totally re-write your code. 64 bit is not the problem though, it's the absolutely horrible hardware driver support, which makes people wait or even move back to a 32bit OS.
Why exactly don't you have to re-write your code to utilize a multi-core architecture?
To truly take advantage of a multi-core operating system, you have to leverage the concurrency features (aka threading model) extensively. A surprising number of applications don't bother. And I don't blame them. Multi-threaded applications are a lot harder to follow and debug.
On top of this, a fair number of multi-core processing systems are poorly implemented with inadequate cache on the CPU. Leveraging concurrency in those situations becomes prone to cache thrashing and actually degrades performance.
If an application is written for a more abstract programming model (such as .Net or Java), it will survive the move from 32-bit to 64-bit with little to no re-write on the part of the developer (depending upon how many times the 32-bit specific APIs are used directly within the app) To convert from a single to multi-threaded app is quite a bit more work in either case.