@Emiya4578
The prerequisites to creating xenos are "death" and "longing" in order to actually create xenos. The dungeon attempted this with the colosseum but failed because it could not succeed in creating the "longing" requirement.
The "longing" requirement can be achieved in a fight between an adventurer and a monster, but it depends on what happens during the fight, and even then, it isn't always guaranteed, but this is how xenos started to come to be in the first place. This is why xenos are so few in number.
The first xenos appeared 30 years ago, which would have been at the end of the Zeus familia and Hera familia era before the OEBD fight, and they were the ones who discovered the xenos first.
My take on this is that evolution is taking its course when it comes to the monsters created by the dungeon. I wonder if that every time a monster is killed it is reborn, but it's a little more evolved and eventually once a threshold is reached and once that particular monster dies again, then the monster "longs" to either live again (perhaps some if not all the other xenos) or face that adventurer (Asterius wanting a rematch with Bell or the Juggernaut wanting to kill him) and is eventually reborn into a xenos. This could explain why xenos have appeared most recently because this process must take thousands of years to achieve.
IIRC, Wiene described having vague memories, but what she was really describing was her memories as a monster before she was killed, most likely by an adventurer under conditions that would have resulted in her "longing" to live again, and thus she was reborn as a xenos.