Why can't I preregister a template variable in a function?

[nerbert]

I got started on a project to make the What's Going On box available on pages other than the home page. The way I wanted to do it is shown schematically below:

  
function whats_going_on($template_name)
{
     global $vbulletin, $show, $template_hook, $vbphrase;
     .
     .
     . 
     $templater = vB_Template::create('whatsgoingon');
     .
     $preregistered['whatsgoingon'] = $templater->render();
     vB_Template::preRegister($template_name, $preregistered);
}

whats_going_on('FORUMDISPLAY');

Well, that didn't work so I ended up doing it like this:

  
function whats_going_on()
{
     global $vbulletin, $show, $template_hook, $vbphrase;
     .
     .
     . 
     $templater = vB_Template::create('whatsgoingon');
     .
     return $templater->render();
}

$preregistered['whatsgoingon'] = whats_going_on();
vB_Template::preRegister('FORUMDISPLAY', $preregistered);

What am I doing wrong that preregistering doesn't work inside a function?