It's probably related to the usage of
all in the expression
all the way (the entire distance; completely) and in expressions like
all gone or
all tired out.
all but X =
all the way it is possible to go in the direction of X-ness,
but without actually reaching X-ness itself.
So,
almost in the state of X-ness,
almost X.
This 'going in the direction of a quality' is, of course, metaphoric.
all but finished =
all the way it is possible to go in the direction of being finished,
but not completely finished = almost finished
all but impossible =
all the way it is possible to go in the direction of impossibility,
but not completely impossible = almost impossible
The paraphrase above is consistent with the fact that the accompanying
adjective has to represent an absolute end point of some kind. It
makes little sense, in ordinary circumstances, to use
all but with
happy or
yellow! (
?all but happy?, ?all but yellow?).
But -- also consistent with the paraphrase -- with past participles of
'verbs of achievement' or adjectives of final state
all but works quite well.
The contest was all but won. (almost won, about to be won)
The matter was all but decided. (almost decided, about to be decided)
The beast was all but dead. (almost dead, about to die)
CJ