Why can't giraffes reach the height of sauropods?

The body size of animals depends on "external physical" factors and "internal physiological" factors. The length of the animals depends on the following factors: 1- Strength of the circulatory system 2- Air pressure 3- Density of Blood 4- Earth surface gravity 5- Amount of hemoglobin affinity for oxygen 6- Body angle relative to the ground Why a today animal cannot be tall like a dinosaur 100 million years ago. Which of the above factors has changed during this time?