Mysteries [1]
The main reason the Abyss remained unexplored was obviously due to its inherent dangers.
From the uneven time distribution where every second could equate to hundreds of years in the universe, to the threatening atmosphere that tried to suffocate one from all sides, it was overall a place that did not welcome the presence of universe denizens.
The worst part of it all was the inability to gather mana. The mana of the Abyss was too corrupted for consumption, so once one's internal mana capacity ran out, only their bodily defenses could protect them from the dangers.
And with the Abyss constantly changing, it was nigh impossible to find one's way back to the universe after entering it.
Unless one had the luck of the entire universe in their hands, it was bad enough to be considered genuinely impossible.
But those weren't the only problems.
There was no motivation for the people of the universe to make their way into uncharted territory.