1491 (or is it the sequel,
1493? I could see this topic ending up in either) makes the case that there's nowhere near as much evidence of Native Americans eating passenger pigeons as one might expect given the vast populations (and incredible ease of hunting) reported in later decades. Instead, the vast numbers may have been a sign of a badly screwed-up ecosystem, with huge swaths of native-managed agriculture and land suddenly going unmanaged, freeing up tons of cheap calories of exactly the kind the birds could use, leading to a gigantic boom in species perfectly-situated to take advantage of it, which would include the Passenger. All this, on account of the continent experiencing an apocalyptic drop in population after European disease arrived (and for other reasons, of course, but the diseases did a great deal of it).
That's not why it went extinct, of course, but does put in perspective what may have been a more "natural" population level for the bird, previously—the shocking decline may have been from an aberrant many-times-larger-than-normal population, not from the range in which the population had tended to stay before extensive contact with Europe. It may also explain why it was possible for it to go extinct so seemingly-easily—they weren't truly thriving as much as one might suppose from the numbers, and in fact were quite vulnerable, especially if people got accustomed to eating lots of them and their population was already destined to rubber-band back to something under its ordinary level.