Guess: warm climate and LOTS of available food.
Lack of predators and continued growth with age, undiscovered lung aids through evolution and forced locomotion by food distribution; still, there’s that pesky problem of mechanics and blood pumping up those long necks required to reach very tall trees.
Elevated atmospheric pressure?
Maybe they simply followed the savanna along the shoreline where the soil was super-efficient at leaching the salt from the intruding ocean underground and in so doing they were constantly at sea level where the ATM was highest.
The O2 levels needed to be at least 21% to prevent lethargy, I would think.
Another question is would there have been mountains where small game could thrive and then foolishly wander to the hunting grounds of the few meat-eaters?
HMmmm...
Where did all the FOOD come from?
After all, LIFE for one creature is FOOD for another, so WHERE did all of the BIOMASS come from?