several canonical reasons
1. when Vegeta says he and Goku are different "Breeds" he means it literally, Saiyans came in specifically bred varieties, with Goku's breed being dumb wall-of-meat soldiers and Vegetas being built to be a leader class and excelling in speed and attack strength as well as intelligence
2. all of Vegeta's transformations are of equivalent power to the form that comes after Gokus, and he skips past several forms because of this. I.e. he cannot use Super Saiyan 3 and his version of Super Saiyan 2 has the same equivalent power as Goku's Super Saiyan 3 form
3. vegeta is in general a more competent fighter than Goku, Vegeta has legitimate honest to goodness Saiyan Warrior training, Goku's strength comes from directionless aggression, so the real reason for Goku's victories is a complete disregard for safety, Vegeta knows how to focus his attacks on an opponant thus delivering ALL damadge to the opponant, Goku just blows up everything around him and that usually works