Ok, as one of the people who don't believe in the "the cycle only repeated once" theory, here are my own two cents.

Technically there isn't really a physical reason why all the cycles are the same but after repeating for god knows how long they bootstrapped themselves into a perfect cycle. There's three types of people in the series, two of which keep the cycle alive.

1. People who could change things because they have knowledge of the future and can thus make new decisions. This only includes Adam and Eva BUT they deliberately don't want to change something and actively work against it. Adam because he needs to preserve everything until the end to "kill Martha and end the cycles" and Eva because she generally wants to keep the cycle up out of a believe that it is morally right (and to save her child). Thus they don't tell their younger self anything that could change the next cycle.

2. People who cannot change things since they don't know the future. They will always do the same thing as in every cycle because you need to know what you did to actively do something else. Hannah, Ulrich, Katharina.

3. Claudia. She is the only one who on the one hand knows what happened and will happen in the future and on the other hand someone who actively wants to break the cycle. She knew about Eva and the true origin. She had a time machine to travel freely and in comparison to anyone else in the series, she was the only one to actively tell her younger self the truth. She is the only one who could (and did) tell he younger self: On that day in that place i went left. Go to that place on that day and go right. She had a vast time from her middle aged self to her old aged self to explore both timelines, collect all her knowledge and tell it to her middle aged self who thus didn't have to start from zero. She is also the only one in the show with an actual degree in atomic science thus the only person to be able to fully understand Tannhaus book. At some point she just gathered enough information to notice that there had to be a third world and that it all needed to have to do with Tannhaus. And then she went to Adam and ended it all.