Why is so little known about Curonian, Selonian, Semigallian, and Yotvingian?

Little is known about Curonian, Selonian, Semigallian, and Yotvingian primarily because these languages left no written records. By the 16th and 17th centuries, the speakers of these languages had been assimilated by neighboring populations—specifically Latvians and Lithuanians in the north, and Slavs in the south—leading to the complete loss of their distinct national identities and linguistic traditions.12

Because these groups did not produce written texts, surviving information regarding their languages is extremely scarce and is largely limited to place-names. While other extinct Baltic languages like Old Prussian left behind some, albeit poor, written documentation, Curonian, Selonian, Semigallian, and Yotvingian vanished without such evidence, making comprehensive linguistic study difficult.12