Mortal kombat tournaments have "always" existed as a safeguard to prevent merging the realms indiscriminately. That's why they put a hard way of winning (10 consecutive tournaments in intervals of 50 years).
We only experience one MK legitimate tournament, and yet it was a corruption of the Shaolin Tournament, which are the events of MK1.
To have another legitimate tournament, we should have waited 50 years. But Kahn sent that attack on Wu Shi Academy, lured guys into Outworld and was kicked, there was no competition at all. Then he invades Earthrealm because the barriers are fiction, there is a rule of not invading and a law of not merging without winning 10 consecutive tournaments.
MK
A events are also not legitimate MK tournaments. It was a torunament, but not a Mortal Kombat Tournament, because it wasn't permitted by the Elder Gods, and the winner would not be granted the prizes. Remember that it was just a fake so the DA could steal the souls of dead warriors.
The same happens on MK4, MK
and even MK:A. "Mortal Kombat" became a title.
Also, Armageddon is constructed by the corruption of MK torunaments with its numerous kombatants, because people are not using them properly. About invasion, it is permitted cause the Elder Gods are truly "toothless worms" and they don't care about invading, people dying, etc. they just get scared if One Being is closer to return. So they put MKTournament so we can pretend we are having any free will about mergin realms.