I think there is no need to resort to conspiracy theories. It is perfectly possible that while he was the best candidate for solo mission, he was simply not the best candidate for a multi person mission. In the Air Force, it has long been known best pilots for single seat aircraftHave a different psychological profile the best pilots for multi seed aircraft.
the “right stuff” for a test pilot, which is what a solo astronaut flying a new capsule for the first time is, is not the right stuff for a multi person crew who must get alone ans collaborate effectively during a protracted mission