Because Dai rabbit is one of the rabbits. Rabbit's tail is very short. So there is a saying in the countryside that children's tails can't grow. Used to indicate that something is coming to an end. Or something can't last long, which aims to show that something is coming to an end.
This is a common saying, a common and stereotyped sentence spread among the people, including proverbs, slang and common oral idioms, and a summary of people's life experiences and aspirations.