As it turns out, Deadpool 2 may have revealed some of the biggest plot points in Dark Phoenix last year, long before the movie ever hit theaters.
That aside, the explanation would seem to be that both Jean and Mystique die in Dark Phoenix and, as a result, wouldn’t be alive when Deadpool 2 takes place.
That said, Deadpool 2 contained a surprising and hilarious cameo from the core X-Men cast, with a couple of notable exceptions.
Either way, this does sort of work now as something that connects well enough to Dark Phoenix.