why some strong AS verbs survived (e.g. grow/grew) while others didn't (e.g. help/healp became help/helped)

Or why some (hang) have both strong (the stockings were hung by the chimney...) and weak (they hanged him at sunrise) forms. For this a grammarian is only going to tell you, "Because I say so, that's why." The linguist will give you the whole thang in four part harmony with circles and arrows and a paragraph on the back of each one telling you what each one is about.