How does preeclampsia causes oligohydramnios?

Decreased fetal GFR. Oligohydramnios is a sign of decreased fetal glomerular filtration rate (GFR) in the absence of rupture of amniotic membranes. Preeclampsia can cause placental failure and thus poor perfusion of the fetoplacental unit leading to oligohydramnios.
Decreased perfusion. Preeclampsia causes decreased placental blood flow which in turn causes decreased fetal blood flow which causes decreased fetal kidney blood flow which results in decreased fetal urine output which results in decreased amniotic fluid volume which is known as oligohydramnios.