As explained very early on, Tess Durbeyfield is descended from the aristocratic D'urberville family. Therefore she is of the D'urbervilles, unlike Alec D'urberville whose family (Stokes) adopted the name.
As to why, probably because Tess of the D'urbervilles
is catchier and sells more books than Tess Durbeyfield.
On the meaning of names this is as good an answer as any.
Names are one of the most important tools for characterization in this novel. Let's start with the most obvious one: Tess's last name. Is it Durbeyfield or D'Urberville, and what are the possible implications of each? Well, Durbeyfield has the word "field" in it, which implies the countryside, and rural simplicity. "D'Urberville," on the other hand, has "ville" in it, which is French for "city." D'Urberville also separates the "D" at the beginning of the name, calling attention to the "Urb-" part of the word. This could imply urbanity (i.e., sophistication), as well as urban, or city life. Besides its rustic connotations, "Durbeyfield" also sounds very Anglo-Saxon, compared to the French-sounding "D'Urberville." Old aristocratic names in England are often French, because of the Norman Conquest in 1066 (check out the Historical Context note in our summary of Chapter One for more on this).