One rumoured reason, would you believe, is
PR. NASA wants the press to be able to cover both…..
Id imagine other shared assets such as the RB-57 and telescope and satellites etc might be a concern.
That's 14:29 UTC for June 6th, right at the end of Starship's window. Even Starliner launching a day earlier will mean lots of spaceflight reporters/photographers missing one or the other.
