Very good question, thanks for asking.
The short answer is that enlightenment isn’t eternal, it’s as temporal as any other mental state. It typically lasts until you die, but I can envision situations where people may completely forget the state while still living and so get re-“captured” by their small self.
This may or may not have happened to Eckhart Tolle. I generally don’t like to make assumptions about people’s mental state, but if you’re looking for a possible example, people using enlightenment to jump off into fame and then getting dragged back down out of the spiritual world and state into the world of ego again, Tolle would be a good one to look to.
That scurrilous bit of speculation aside, think back to the genesis of ‘you’. Birth. Your life had to be created, your memories and patterns and literally everything that causes you to think the way that you do about yourself and others. You can dive back into events of your childhood and pick out pivotal moments where you had convictions that changed the way you viewed the world.
Enlightenment is just one more of these events. Only it doesn’t happen in your childhood, it happens usually in adulthood. You start reading about spirituality and start naturally gravitating towards the path of surrender. The path attains a life of its own and you soon find yourself carried along this path to the ultimate surrender. You push through one day through the Dark Night to find the ‘final’ samatha.
Very very common, lots and lots of people go down this road, or to be more clear, the road drags them along it. If it’s so common, why do we insist on it happening only once in a being’s existence?
It’s not. A lifetime is only one unit of existence. Enlightenment is something you don’t just attain, it’s something you also have to master. You don’t master something by doing it just once. You need to keep doing it, over and over again, until you’re familiar enough with the forms and ups and downs and variations and inversions. Only then can you recognize that there’s no one path to enlightenment, that all the religions point to the same ultimate reality.
Once mastered, enlightenment is eternal. Mere attainment is just as temporal as everything else.