You can add unofficial software sources like PPA's, to get more bleeding-edge software. The software you see in the Software Center is whatever is provided by the software sources you have enabled.
So this question isn't really about the Software Center itself, but instead about why the official software sources, enabled by default in Ubuntu, don't provide the latest and greatest versions of software as they come out.
This is because of Ubuntu's policy of maintaining a stable user experience, and predictable functionality, within a release. When an Ubuntu release comes out, updates are not released to add features, or even to fix minor bugs. Instead, they are released only to fix major bugs (or any security vulnerability).
There is a small exception to this for Long Term Support releases, to support new hardware, provided that doing so doesn't break any existing functionality.
The community documentation on Stable Release Updates explains this policy in detail, including its rationale.