More ‘Kryten’ and Less ‘Replicant’: The Case for the Mechanical Tell

In Star Trek, why does the computer sound so mechanical and emotionless when the holodeck can create characters who can converse indistinguishably from humans? In Voyager, the Doctor himself is as realistic as it gets, complete with a receding hairline and a snarky attitude. Why does Data have yellow skin? Why is he apparently (though not consistently) unable to use contractions... except when mimicking the speaking style of Sherlock Holmes? Of course, these sci-fi characters are all fictional...