Dating back to the original Dragon Ball, Saiyans have been feared not only for their strength but also because they transform into giant Great Apes when the moon is full. Up until the Saiyan Saga, we see Goku, Gohan and Vegeta transform into these King Kong lookalikes but after that, never again. It's rumored that creator Akira Toriyama just forgot about the tails, so Goten and Trunks never had theirs. But what are some in-universe reasons we can use to explain this?
Goku causes mass destruction and even the death of his adoptive grandfather, Gohan, when he transforms. Goku had his tail cut off a total of three times throughout the original run of Dragon Ball. His son, also named Gohan, would go on to inherit this trait, though Chi-Chi and Goku make sure he doesn't see the moon. It isn't until Piccolo takes him out into the wilderness that he sees the moon for the first time, and thus transforms into his Great Ape form, much to Piccolo's surprise. Piccolo then destroyed the moon and yanked the tail off.
In the anime, it grows back and Piccolo once again removes it, but it grows back during the fight with Vegeta (both in the anime and the manga). With the exception of GT, which many fans don't consider canon, the tail never makes an appearance again.
The Daizenshuu guide series clarifies that a tail will grow back if the Saiyan is in immediate danger, as we saw with Goku in Dragon Ball. The guide books were originally printed in 1995, with an updated, more condensed version released in 2013. In Dragon Ball Z: Kakarot, it's explained that when a tail is cut off, there's an eight percent chance for it to grow back and that it won't grow back at all if the Saiyan surpasses the power of the Great Ape form without having transformed into it.
Therefore, if a Saiyan reaches Super Saiyan without being in Great Ape form, the tail won't grow back once being cut off. If they reach Super Saiyan while in Great Ape form, the tail has an eight percent chance to grow back. Since the latter information came from a video game released this year, it isn't clear whether this information is considered canon or not.
A scientific basis for this could be that since both Vegeta and Goku married humans, the trait became recessive, and Gohan just so happened to have inherited it, while Trunks, Goten and Bulla didn't grow them. If you have brown eyes and your spouse has blue eyes, then you have four children, there is a 25 percent chance that one of those children will have blue eyes since they are recessive. Since there are four Saiyan-human hybrid children, it would make sense that at least one of them would have a tail if the gene became recessive. Gohan, the oldest, just so happens to be the one to inherit it.
Fans on Reddit also speculate that the younger children did, in fact, have tails, but Chi-Chi and Bulma had them removed to prevent them from transforming into the Great Apes. During the Buu Saga, Trunks and Goten were able to reach their Super Saiyan forms at a very young age, so it makes sense that they would not regrow their tails once that had reached that point. This doesn't explain why Bulla didn't have a tail when she was born, but the trait could have been recessive in her case.
While the consensus is Toriyama just forgot to draw them on the kids, there are several in-universe reasons why they don't appear again after the Saiyan Saga that fans can consider. It could be science, it could be new lore, or it could just be headcanons. But the consensus is that the tails were a hindrance to the Saiyan's growth in power, so maybe it's better that they're gone for good.
