Pumpkins have become unavoidable during the Halloween season, especially in North America, for carving jack-o'-lanterns. But interestingly, the poor (but fat or obese) pumpkin got into the fray of a competitive Halloween add-on accidentally. It had no role in Halloween in the early history of the celebrations.
But, other vegetables such as the turnip, mangelwurzel, or swede (a type of Swedish turnip) were traditionally used for carving lanterns throughout Britain and Ireland. Also, until 1837 jack-o'-lantern did not appear as a term for a carved vegetable lantern, and the carved lantern did not become associated with Halloween until 1866.
As carving jack-o'-lanterns and use of it as a Halloween lantern occurred in not in Britain or Ireland, but in North America, Historian David J. Skal writes: “Although every modern chronicle of the holiday repeats the claim that vegetable lanterns were a time-honored component of Halloween celebrations in the British Isles, none gives any primary documentation. In fact, none of the major nineteenth-century chronicles of British holidays and folk customs makes any mention whatsoever of carved lanterns in connection with Halloween. Neither do any of the standard works of the early twentieth century.”
It just happened that in the United States, the carved pumpkin was first associated with the harvest season in general, long before it became an emblem of Halloween. It has also been recorded that pumpkins were easy to cultivate in the United States, and the size of them were significantly larger than their cousins in the British Isles or Europe, where the Celtic festival has its roots.
Firstly, because of their large sizes and cheap prices, it was easier to carve lanterns on pumpkins than on the traditional turnips or swede. So, someone probably experimented with the poor plumb pumpkin and others followed suit. And they, instead becoming an alternative, became the main vegetable for carving jack-o'-lanterns.
And then, there was the reverse flow of culture and tradition, as the old West started aping the New West, and now in European countries and other areas where Halloween is celebrated the Pumpkin is the King of Vegetables during the Halloween season!