Why does labor take such a long time for first-time moms?

It does not always. Take a long time, I have had pts push the 1st baby out in 5-15 minutes while other women took 2 hours, once u have had a vag del if the 2nd baby is not any bigger it usually takes less time because the vaginal canal has been stretched out from the 1st delivery, but sometimes the baby can get stuck so your health care prof should be skilled enough to know that and when to do a section if necessary.
Lack of stretch. The ligaments of the pelvis as well as the "stretchiness" of the cervix is relatively tight in women who have not delivered a child vaginally. Once you have delivered a child vaginally then these ligaments and the cervix are looser and stretch faster and more easily with the next pregnancy.