The answer Brad received was somewhat unbelievable, yet it made perfect sense.
Because this Doctor Doom alone was the Doctor Doom from Earth-616.
Before the original Reed Council disbanded, Mister Fantastic of Earth-616, Reed Richards, had used some method to establish a rule.
"Only people and things from Earth-616 can truly destroy one another. No other universe may interfere with anything in Earth-616."
That left Reed Richards from the Ultimate Universe powerless. Although he called himself the Creator, that title only held sway over universes other than Earth-616 and Earth-1610-or, rather, over the people and things within them.
Ultimate Reed could do nothing to God Emperor Doom, who came from Earth-616, so he could only keep him imprisoned.
Likewise, that very rule was why Brad had been barred from Earth-616. After countless attempts ended in defeat at the hands of Earth-616's Reed Richards, the Creator had added another lock to the existing one to prevent any "bacteria" from successfully sneaking out of the universes under his control.
With the two layers of "rules" established by the Reeds of Earth-616 and Earth-1610, Brad had been erased the instant he passed through the Portal during his earlier attempt to sneak in.
"But why could Kang get in?"
Brad suddenly remembered that Kang the Conqueror had successfully slipped in before!
"Because Kang the Conqueror comes from the future, and there is no Reed Richards in the future. Or, rather, the universe's future state differs from its present one."
God Emperor Doom was surprisingly patient. After all, he had been imprisoned for a long time, and the Creator had thoroughly ground down his former arrogance and rough edges.
"Kang's state is very strange."
Doom considered his wording. "At the very beginning, there should actually have been only one Kang: the Kang of Earth-616. All