Why do women have to go on bed rest in pregnancy?

Bed rest. Often, it's because they are at risk of losing the baby. A common reason is because of an incompetent cervix, where the cervix prematurely opens and could allow baby to descent too early down the birth canal. Other causes could be the connection of the placenta to the uterus or it's location. There are many more reasons, but these are a couple of common ones.
Lots of reasons. Patients experiencing a normal pregnancy can stay active and do not need to be on bedrest. Some complications require bedrest. Some complications include preterm labor, ruptured membranes (water broken), a short cervix, a dilated cervix, high blood pressure, placenta problems and vaginal bleeding.