I've recently finished watching the last kingdom on netflix and most characters even the christian ones have names like "whitgar, Uhtred, Ethelfled, Ethelstan,etc. " these name ssound like they're out of beowulf but if you look now at england these names are non existent and most people have the names we now know like john, chris, mark, etc. Is this because of the Norman conquest? Or is this simply due to the passage of time and the changing of language?
So there are few things to get at here.
First off. Not all Anglo Saxon names have totally vanished! Carl, Edward, Edgar, Edith, Harold, Wilfred, and Mildred are all modern English names that are directly descended from Anglo-Saxon names. Many of them are somewhat rarer these days, but not unheard of, and they were far more popular not that long ago. These dont always resemble Old English spellings because English has changed a good bit in the intervening centuries. For example, Ælfræd becomes Alfred following changes in pronunciation and the Great Vowel Shift of the later Middle Ages. So while we dont have too many Æthelwæld's being named these days, not all of the names have vanished!
However there was indeed a shift in noble names following the Norman conquest and this has gradually shifted down to the English speaking populace at large. Names such as John, Wiliam, Anne, Thomas, Jeffrey/Geoffrey, Rebecca, and Elizabeth, of which some are Romance names as well as names brought into French from Greek or Hebrew roots (usually through the Bible or as names of Saints or other prominent religious figures, indeed all three names you mention arrived in English this way) were introduced on a much larger scale following the Norman conquest. Now some names that are common under different spellings today, Robert for example, do have Old English cognates, but the modern spelling is derived from Romance antecedents (it was exceptionally common among Normans)