Tebow didn't make their running game great. That's 90% scheme. Any team with a QB that ran the spread option in college could run that same offense if they wanted to. Teams don't do it because they don't believe it can work long-term. The Broncos don't care because they don't plan on having Tebow as their starting QB long-term. This is just a holdover until they find a real QB.
Why's it working now? I'd argue that it's not working. They've only scored 20 points once since Tebow took over.
If you believe that it is working: The Broncos started running this offense halfway through the season. No one was prepared for it. I don't think there is a single DC in the NFL right now that has ever schemed against the spread option. If Denver announces that they're keeping Tebow, all of the NFL coaches will be calling up their college buddies during the offseason to help them design schemes to defend the Broncos offense.
Monte Kiffin is currently USC's DC. In his first two years, USC's defense was terrible against Oregon and the other spread option teams. This year, they beat Utah and Oregon.