Force mac to show at least two updates using softwareupdate -l
I'm pretty sure that softwareupdate is only going to show you the most recent update. It wants to update you from what you have to what's current, and it doesn't show other updates that might have happened in between.

If you want to download past versions of Xcode, you should go to Apple's developer site, where every version of Xcode since version 3.0 in 2007 is available. If you have a developer account, you can get them at http://developer.apple.com/download/more/. If you don't currently have a developer account, you can sign up for free at https://developer.apple.com/programs/