This is my speculation.  When the Sayin blood mixes with humans, the resulting babies are hybrid and have a lot more potential than the original Sayins. That's why Gohan was pretty strong for his age. Coming to Goten and Trunks, they were born when Goku and Vegeta had the super Sayin transformation. The younger generation was a better  hybrid of Sayin and human, and thus they are able to achieve it at a phenomenal rate compared to Goku/Vegeta