Why does damage to the cervix cause fertility problems?

Cervical mucus. When patients have traumatic procedures done on the cervix, such as a leep procedure for abnormal pap smears, it can damage the mucus glands. Cervical mucus is essential as a "conduit" for sperm to travel from the vagina to the uterus, and to "cleanse" sperm of extraneous elements like bacteria and white blood cells. Also, the mucus helps the sperm to be able to fertilize sperm.
Cervical stenosis. Also, procedures such as cone biopsies of the cervix and cervical freezing for abnormal cells or inflammation can cause the opening of the cervix to stenose, or become very tight, which can also alter the production and expression of cervical mucus.
Incompetent cervix. Rarely procedures such as d+c's or leep cone biopsy can weaken the cervix, making it unable to retain a pregnancy. Women with this problem usually miscarry late, after 16 weeks.