Why do linguists call the broader Tai language family Tai-Kadai or Kadai?

Linguists use the terms Tai-Kadai or Kadai to describe the broader language family that encompasses the Tai languages and their various linguistic relatives.1

The classification reflects the inclusion of several distinct groups within this family, most notably the Kam-Sui languages of Guizhou, China, and the Li (or Hlai) languages of Hainan. By adopting these labels, scholars categorize the Tai group alongside these related languages, moving away from older, now largely rejected theories that attempted to classify Tai as a division of the Sino-Tibetan family. While some researchers have proposed a potential link between this family and Austronesian, that connection remains unestablished to the satisfaction of the broader linguistic community.12