Originally, hollowed-out turnips and beets were used in Europe to make "Jack's lanterns." After this practice reached North America, pumpkins were found to be high-yielding, large, and soft inside, making them easier to carve and light, so they became the standard. Today's jack-o'-lanterns are mostly porch decorations rather than items meant to ward off evil.
Time and Origins
Why is a jack-o'-lantern made from a pumpkin?
Because pumpkins are bigger, softer, and easier to carve than turnips, North American immigrants replaced the Irish turnip lanterns with pumpkins.
Short answer
Because pumpkins are bigger, softer, and easier to carve than turnips, North American immigrants replaced the Irish turnip lanterns with pumpkins.