That the forward triangle is bisected and the rear triangle made of thinner tubes may have tended to isolate jarring from rough roads to the front half, while giving a small amount of compliance to the rear half.
The abbreviated seat tube may have helped as well, being mounted to skinny tubes and too short to transfer shocks from the main tubes. Less likelihood of pesky harmonics...other than the kind that you get from overdoing the bangers and mash, at any rate...