The first jack-o’-lanterns were carved from turnips because that tradition began in Ireland, Scotland and other parts of the British Isles, where pumpkins were not grown. Root vegetables such as turnips, and sometimes potatoes, radishes or beets, were plentiful after the harvest, and metal lanterns were expensive, so people hollowed them out, carved faces into them and put a candle or ember inside. The carvings were set outside homes or carried as lanterns during Samhain, the festival that begins on the evening of October 31, to ward off wandering spirits such as the folk figure Stingy Jack.
Turnip lanterns were still common in Ireland in the 19th and early 20th centuries. Irish and Scottish immigrants brought the custom to the United States, where native pumpkins were larger and easier to carve, and pumpkins became the standard Halloween jack-o’-lantern.
Source: Smithsonian Magazine: When People Carved Turnips Instead of Pumpkins for Halloween (see also HISTORY: How Jack O’Lanterns Originated in Irish Myth)
Image: Traditional Cornish jack-o’-lantern made from a turnip, by Bodrugan, CC BY-SA 3.0, via Wikimedia Commons.