Why Were the Ackermans Persecuted?

However, along with the Asian clan, they rejected the King's idea of peace, and turned their backs on the monarchy. Fearing his inability to control them, the King persecuted them, bringing both races to the brink of extinction.

Why can't Ackermans be Titans?

As mentioned above, the Ackermans are immune to the power of the Titans -- their memories can't be erased by the power of the Founding Titan. The reason for it is that their bloodline has been modified, as they are no longer considered as Eldians who can turn into, and/or affected by the power of the Titans.

Why are the Ackermans so powerful?

Thanks to an "awakening power," which gives them physical strength far beyond humans' normal capabilities, the Ackermans are able to channel the fighting power and experience of their ancestors, essentially utilizing the strength of Titans without becoming one themselves.