If we're to bring in DBZ Kakarot, that explains this as there being an 8% chance of a tail regrowing when cut, making it essentially a gamble if it grows back or not. It also makes it apparent that Saiyans become unlikely to have their tail grow back when their power reaches a certain threshold (ie. getting close to achieving Super Saiyan under the right conditions) which makes sense when both tails and S-Cells are genetic traits and those with a naturally greater amount of the former such as Goten and Trunks or the U6 Saiyans were born without them.