The case of some sites including ours is different. It shows beta in main site also.
What is the difference?
I asked this on Meta SE here: Are the 'Quantum Computing' and 'Artificial Intelligence' sites still in Beta?.
According to Glorfindel's answer, the lack of a Beta label on the AI site is most likely because the site had a sponsorship. There are other sites like this as well, and the details are covered in the Meta question and answer.
See these posts on Meta SE for which sites have had sponsors and what a sponsorship entails: