The absence of the Central Line at TCR may be because the interchange there isn't particularly easy compared with Liverpool St and Bond Street. Moreover, Crossrail passengers for the Oxford Circus area can use the Eastern exit at Bond Street station, which is in Hanover Square (or the Dean Street exit from TCR). No need to use the Central Line.
For Holborn it's probably easier to change at Kings Cross rather than double back from TCR. And for Chancery Lane and St Pauls it's quicker to walk from Farringdon.