It can also map unto "maybe", which opens another can of worms.
Not that there's anything wrong with having an extra value "unknown", but it doesn't fundamentally alter the logic. As unknown in most cases means "it will be true or false at some point", its usage in computing is that of a singleton (who needs a word with 64 potentially unknown bits?), so dealing with unknown values as an exception is easier than permeating hardware with it. Using ternary to represent unknown is just not efficient.
Not that there's anything wrong with having an extra value "unknown", but it doesn't fundamentally alter the logic. As unknown in most cases means "it will be true or false at some point", its usage in computing is that of a singleton (who needs a word with 64 potentially unknown bits?), so dealing with unknown values as an exception is easier than permeating hardware with it. Using ternary to represent unknown is just not efficient.