It's about weight and torsional rigidity. Just like the power to weight ratio is a more relevant number than just the straight up power figure. The rigidity to weight ratio is more relevant than just the rigidity factor alone.
The monocoque chassis has more rigidity for a given weight, however as already stated it is more expensive and difficult to build and harder to repair.
The original car had 12,000 lbs/degree torsional rigidity, the tube frame cars don't begin to approach that. The best are about 1/2 of that. That said the only place you'll notice is going 9/10 or 10/10 on the track, and even then you will have to be a developed driver to do so.