The biggest reason was over hunting, but deforestation also contributed. The Passenger Pigeon was the largest species of birds, during the 1800's. It contained more individuals than all other North American birds combined. But sadly, by 1900s that powerful population had been reduced to one bird named," Martha" who died on September 1, 1914 in captivity at the Cincinnati Zoo. At one time a single flock of passenger pigeons might have more than 2 billion birds. That is just one flock, there were many flocks in the US at the time of this bird's peak of abundance. But, unregulated commercial hunting, specifically for their meat, on a wholesale scale, used those large billion bird flocks to man's advantage and it devastated the birds populations. The species migration and nesting habits also made them easy targets for large number kills, either by netting, torching, or special firearms, even one, the forerunner of the machine gun. By 1855 one year in Michigan a billion birds were, so called "harvested". The passenger pigeon's population, of course suffered a collapse under the strain, and could not recover. There were no protection measures, no conservation lands. The pigeons left were scattered across the US and that made breeding difficult and the numbers continued to decline. By 1900 all wild passenger pigeons were extinct. Now, with today's knowledge, some believe that the species should have been able to recover from the uncontrolled commercial hunting, but the scattering of the birds may have been the final straw. The large flocks may have been part of their rituals for breeding and once scattered these rituals were not triggered, therefore the breeding declined and the species suffered extinction. For more details, please see sites listed below.
The passenger pigeon went extinct because of hunters over hunting the animal and habitat loss.