I understand they had greater power than most lords as they were there to control and extend Anglo-Norman power, but why were the areas they ruled known as the Marches?
Because "March" is an old word for boundary or border.
march (n.2) "boundary," late 13c. (in reference to the borderlands beside Wales, rendering Old English Mercia), from Old French marche "boundary, frontier," from Frankish *marka or some other Germanic source (compare Old High German marchon "to mark out, delimit," German Mark "boundary;" see mark (n.1)). Now obsolete. There was a verb in Middle English (c.1300), "to have a common boundary," from Old French marchier "border upon, lie alongside."
This use of "march" means "boundary":
"boundary," late 13c. (in reference to the borderlands beside Wales, rendering Old English Mercia), from Old French marche "boundary, frontier," from Frankish marka or some other Germanic source (compare Old High German marchon "to mark out, delimit," German Mark "boundary;" see mark (n.1)). Now obsolete. There was a verb in Middle English (c.1300), "to have a common boundary," from Old French marchier "border upon, lie alongside."
hi! if you're interested in diving a little deeper, here are a few previous posts on marches