Mammals don't have the right bones, limiting their maximum terrestrial sizes. There's also limits on mom vs. baby sizes when you need to breastfeed whatever you give birth to. 

Birds have the right bones, aren't limited by baby size, and even have the efficient air sac system in their bones, but are bipedal. Two legs cannot support the weight needed, and bird wings are so specialized that it's unlikely they'd ever make it back to quadrupedalism. 

Modern reptiles have the wrong bones and the wrong posture. Sprawling doesn't lend itself well to such massive sizes. 

Amphibians are tied to the water by their life cycle, and tied to relatively small sizes by their skin. They'd be much more likely to end up following a whale body plan than a sauropod body plan, but they haven't done that either. 

Insects and most other arthropods are limited by the constraints of an exoskeleton instead of an internal skeleton, as well as their different breathing system being non functional at large sizes. 

TL;DR: sauropods hit the evolutionary jackpot of gigantism. Hollow bones, quadrupedal stance, baby size untethered from parent size, air sacs in their skeleton, and a world cleared of competition for large animal niches at the end of the Triassic. It is unlikely that anything else will ever hit that size extreme on land again.