As per the DTI books, alternate universes can be created, but they need a huge change from the "prime" timeline to stop their waveforms collapsing back into it. I.e, the human physiology difference Discovery added the MU. That universe was unfixably changed as soon as that mutation took root. The Kelvin verse too, the destruction of the Kelvin hugely altered the life of one of Starfleet's most famous officers and saw the destruction of one of its founding planets.

The Mirror Universe is some kind of true branch that happened much earlier, such that even the physics of light are slightly different.

Kovich’s explanation of branching universes suggests that they’re not common but also that as they increasingly diverge, it becomes less and less possible to corpse over and those who do may not be able to survive.

He noted that no contact had been possible with the Mirror Universe in several centuries.

The 24th century Kelvin Universe officer who came over to the Prime Universe in the 32nd century died, as would have Georgiou. But the Kelvin Universe was close enough that he was able to cross at all.