First thing to consider is body heat. The Elephants of today have giant ears, for the express purpose of shedding heat (plus apparently they get hard of hearing in old age?). Since there’s no evidence that dinosaurs had any similar heat-removal mechanisms, we need to conclude that although they may have had some heat-generation mechanisms (latest pet theories), they were generally not fully warm-blooded.
With warm-bloodedness, as alluded to, comes a host of other issues. The animal can still move well when the climate is cold, but also needs significantly more food and the protective mechanisms (fur or blubber or feathers) to preserve that heat in cold climates. So consider a croc or gator - a cold-blooded animal can grow from a tiny finger size to a dozen feet or more, and with minimal change of body morphology or adaptions (other than relying on a warm climate). A creature that starts off the size of a mouse and grows to twice the size of an elephant, warm-blooded, will have to go through a significant change in heat-adaption strategies. A body twice the dimension of an Elephant generates 8 times the heat with 4 times the surface area to shed that heat.
Another factor is brains. The human brain is the single biggest factor in our survival, despite the evidence of some specimens. Smart outdoes strong very often. The Jurassic may have been an arms race of size and armour, but the last 65 million years appears to have been a brain race.
I have read that the human brain consumes 30% of the body’s calorie intake. To feed brain and size, any warm-blooded animal with a decent brain would have to eat a massive amount of food. Grazers like cows and such are non-stop feeding machines, since the nutritional value of grass is quite low - even when using bacteria to aid in converting cellulose for digestion.
But, warm-blooded predators are very active - so slow, ponderous, massive cold-blooded animals would not have the speed or smarts to avoid becoming a meal; ditto for their small-brained tiny offspring, before they have a chance to become large. Most surviving reptiles have occupied niches to marginal or complex for mammals - swamps and tidal shorelines, isolated islands, or moving in and out of water.
Mammals tend to have few offspring, and the larger the mammal, the longer the gestation and child-rearing cycle. Other than relatively primitive marsupials, offspring tend to be not that much smaller than adults by the time of their birth.
So to sum up - mammal morphology doesn’t allow larger beasts for heat and food reasons. Smart, fast, active predators have killed off anything too big to get away, or picked off their young.
So what about mammoths? Read Darwin’s account of the Galapagos. The first humans to get there found the birds, adapted to the isolated island, so unaware of the danger of humans that one passage describes a sailor pouring a glass of water, and a bird lands on his arm to start drinking it as it pours. A visitor a decade or two later describes a young boy with a 3-foot stick sitting near a watering hole, whacking birds and killing them for dinner. Another few decades or many bird generations later, and the surviving birds are as skittish as the mainland. The same applied to the dodo - too unaware to be frightened of humans, until it was too late. Mammoths would have been even worse. by the time a few generations of mammoths learned (were selected) instinctively to fear humans, there weren’t any more generations.
This is the pattern everywhere outside of Africa and Eurasia - animals had to learn the hard way to fear humans. If numbers were too few and generations too long (or humans too mercenary) then sayonara. It wasn’t that anyone decided to hunt them to extinction; just that every individual actor fed themselves in the most expedient way with powerful weapons developed to feed themselves when prey were much less cooperative; and a large prey would continue to not be aware of the danger of humans long after the smaller ones had evolved to figured out the run away, so they were targets of convenience.
As for elephants, they evolved alongside humans and so learned to avoid them before they had more lethal weapons of mammal destruction.
)