could you tell me why the dancestors are called that?

It’s an abbreviation of Descendant and Ancestor.

“Due to the complexities in familial relationships from each player group considering themselves the descendants and their counterpart player group the ancestors, causing confusing terminology, the term originated as a portmanteau of descendant and ancestor as a single simple term to describe the relationship. It is likely that Porrim was referring primarily to troll players, possibly as a reference to the fact that it is not apparently known which set is the biological ancestry of the other.

By contrast, for the humans, it is known that the B2 kids are the parents of the B1 kids. However, as the term appears to mostly deal with the "perceived” direction of the relationships, it could be seen as applicable to the humans as well; as few of them are aware that their grandparents/ancestors are their children. Roxy has started to catch on to it after seeing Rose in a dream bubble for the second time. Jade has long since been aware that her “pen pal” is her grandson, but she has never indicated awareness that Jake is also her grandfather.“

~Mod T