Why did his career drift off?

I don't think that the quality of his work faded away...
The quality of his work is "hors concours" but after Back to the future series a lot of expectations was set to him and his career and nothing had came up with the same quality.
DOC character for him is like Harry Potter to Daniel Radcliffe, extremely difficult to leave it aside and return to the an ordinary path.
Lloyd is Doc, just like that. And that is really amazing, a job (a character) for a lifetime, he doesn't need to prove anything else.
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I noticed him doing some voice work for a PBS kids show, along with another high-profile has-been.
God love these people. Hopefully they have made their $$ and now do what feels good to them.

I saw an old episode of law and order: criminal intent with Christopher Lloyd guest starring. I love both of them, but that episode was awful 😐

He was pretty good in a movie from 2016 called 'I Am Not a Serial Killer'.

Most beloved actors? Aside from One Flew Over The Cuckoo's Nest and Back To The Future what else has he done that's any good?

Do you really need to ask such a stupid question???😂

He got famous and hit his stride when he was already old, so he had a short self life. Cuckoo's Nest kick-started his career and TAXI made him a household name. By the time BTTF3 came out he was already in his mid 50s so there wasn't much left for him.

He's had some good supporting roles, but it's clear over the last decade especially his age is very apparent and he really can't be expected to act in a major capacity.

Llyod was a 2 trick pony. A rare exception in Hollywood.