This is because the node-view is cached. (ViewCaching=enabled). This is because is should not be needed to regenerate an article (for example) every time it loads.
The caching system handles view parameters (on the format /myurl/(myparameter)/myvalue. Where it is available in the template as $view_parameters.myvalue. I know there are extensions for transforming get or post variables to view parameters. I could however not find it right now.
Alternatively can you disable the view-cache for your specific template. This is done by adding the following to the template