The major cause of extinction of Steller’s sea cow and passenger pigeon is

The major cause of extinction of Steller’s sea cow and passenger pigeon is overexploitation.
Steller’s sea cow (Hydrodamalis gigas) was a large marine mammal that was endemic to the North Pacific Ocean. The Steller’s sea cow was hunted by humans for its meat, hide, and blubber, and its slow reproductive rate made it especially vulnerable to overhunting. The native people of the region hunted the sea cow for centuries, but their hunting practices were sustainable and did not cause significant population declines. However, when Russian fur traders arrived in the region in the 18^th  century, they hunted the sea cows on an industrial scale, driving the population to extinction within 27 years of their arrival.
The passenger pigeon (Ectopistes migratorius) was once the most abundant bird in North America, with flocks numbering in the billions. However, the population of passenger pigeons was drastically reduced by human activities, especially hunting and habitat destruction. Passenger pigeons were hunted for their meat, which was considered a delicacy, and their feathers, which were used for pillows and decoration. The last known passenger pigeon died in captivity in 1914.