Why was there only 9 aware titans with powers?

Hello everyone, I’v watched AOT since it was released till the last episode, I read the last chapters of manga when anime ended too, yet there is still this question that I didn’t find answers for which is why is there are only 9 smart titans and the rest are basically stupid who lives forever?

AOT has the best writing I’v come across so far and it never leaves something unanswered, it’s explained the past using the future and vice versa and never leave a plot hole, even in early episodes there were hints about the future, nothing was left as a coincidence.

Yet this is the only unanswered question that I hoped there was an explanation for while I was watching S4.

The 13 years curse was cuz Ymir lived for 13 years before she died, but why there was only 9 smart titans that has unique powers which can be inherited but the rest are dumb? Why did the number stopped here? And how did they gained their different abilities like attack titan ability to see future, or armor titan dense sheild..etc, what was unique about their spinal fluid that the 10th person just didn’t get power even though the source of all the spinal fluid was originally from Ymir?

What part in history where holders got smart powered titans that reached the 9th titan and boom it just stopped, and everyone else was stupid titan. And why 9?

I tried to search more in the manga and the series but I couldn’t find an answer so I figured maybe someone here knows.