The reason? First twins often deliver early.
Second, something called twin to twin transfusion where one twin gets more nutrition than the other.
Third: the twins get tangled or have an abnormal lie (sideways).
Four: after the first twin delivers, the second might go sideways and not deliver. Or the placenta might detach from the uterus getting smaller. Or the umbilical cords might tangle up.
Esau and Jacob's delivery in the Bible tells the story of on such delivery. Luckily they must have had a skilled midwife, because it can result in a dead mom and dead babies.
Usually if there is a problem they do a Cesarean section, but in the olden days they would go in and turn the baby to deliver it.
A story from Africa: One of our sisters was called to help with a home delivery that wasn't going well. She delivered baby number one but then a hand came out, and she wasn't sure what to do (she wasn't a nurse but a teacher). So she put the hand back inside, and told the family it was time for her to pray; she went home and checked the books on how to turn the kid around for delivery, and then rushed back and delivered the second baby. It was a boy, who later became a priest and always called her Grandmother....
My grandmother's great-grandmother was a midwife in a village in Europe in the mid-19th century. She didn't deliver my grandmother's twin sisters (she lived in a different village, and was no longer practicing her profession by then--she died about a month after the twins were born).