The out of universe answer is probably that Tolkien probably very much enjoyed making up these names, as an exercise in playing with and building his languages and world. He likely would have only fallen back to numbers either when truly and deeply stumped (I don't think this really would have happened often), or to signal something.

I also believe you can also see some of the Old English influence on Tolkien here. If you look at the Anglo-Saxon Kings, you see relatively little re-use of names. Instead (going off the top) you get Egbert, Ethelwulf, Ethelbald, Ethelbert.