Pumpkins became the standard for Halloween lanterns largely due to the Irish and Scottish practice of carving faces into turnips or other root vegetables during Samhain, then the tradition shifting to pumpkins once Irish immigrants brought it to North America where pumpkins were more abundant. The core idea was to ward off spirits and guide the living during the dark season, drawing on a mix of folklore about Stingy Jack and practical lantern-making. Key points

• Origins in Celtic and Celtic-adjacent autumn rituals: carving lanterns to ward off spirits during Samhain, originally with turnips and other hardy roots. This served both a protective and guiding purpose for travelers at night. [web results referencing Samhain and lanterns]
• Stingy Jack legend as a shaping tale: a demon/Devil pact story that ends with Jack doomed to wander with a lantern, often depicted as a carved turnip or similar vegetable. This myth helped popularize the “jack-o’-lantern” concept. [web results referencing Stingy Jack]
• Transition to pumpkins in America: Irish immigrants found pumpkins easier to carve and more plentiful in the New World, turning the classic lantern into the pumpkin-centered tradition we see today. [web results noting the pumpkin adoption in the U.S.]
• Modern practice: pumpkin carving became a widespread Halloween activity beyond rural folk customs, evolving into designs ranging from scary faces to intricate artwork, with candles or LEDs inside to illuminate them. [web results on contemporary practice]

If you’d like, I can pull specific historical references or provide a concise timeline with cited sources for each of these points.