Implicitly converting strings into bytes or vice versa means now all your APIs grow an exception "Invalid encoding" / "Can't encode this" / "Can't decode this" / etcetera that you need to deal with.

Making people actually do the conversion has the advantage that when writing their string conversion code they might actually do something with the exception beyond maybe logging it and then pressing on anyway. It also gives you the opportunity to explicitly offer them alternatives like treating everything we can't encode as some sort of replacement character (works well for Unicode, not so much for ASCII), which is way too much to ask of every single function that takes bytes.