Why do pandas typically only raise one baby if they have two?

Half of all panda births result in twins.

It is most uncommon for both cubs to survive infancy as giant pandas will nearly always abandon a cub if they give birth to two as they don't have sufficient milk or energy to care for two. They focus on only the strongest club. Rearing a cub is difficult for pandas because they rely on a bamboo-only diet, which is low in nutrition. Mother pandas need to balance their own survival with that of any cub.

At the Chengdu Research Base of Giant Panda Breeding, when a mother panda gives birth to twins, the research team employs sleight of hand to keep both cubs alive. They do this by removing one of the cubs, so give the illusion to the mother that she only has one baby. The twin panda are then switched up to 10 times a day to maintain the single panda illusion. One cub is with the mother almost all the time while its sibling is kept in an incubator and fed formula milk.

https://www.bbcearth.com/news/the-panda-who-didnt-know-she-had-twins