As Dragon Ball Super introduced the concept of a multiverse to the official Dragon Ball canon, Goku and his friends were revealed to be living in Universe 7. The anime series' climactic story arc then sees the Omni Kings bringing the multiverse together for the Tournament of Power to determine which realities would survive.
While Universe 7 emerged victorious by the end of the galactic contest, their world was surprisingly viewed as the weakest in regards to the development of its mortals' power level. Given this revelation, the reasons behind the Z fighters' universe being far behind its counterparts in development terms may have been quietly explained in Dragon Ball Z.
The final major antagonist in DBZ was Majin Buu, a monster created by the evil sorcerer Bibidi eons before the main events of the manga/anime series take place. In the span of several years, Bibidi led Buu to destroy hundreds of planets and kill millions who were tragically caught in the duo's path. Two of the five Supreme Kais were killed by Buu while two more were absorbed, triggering a transformation. As Buu became harder for Bibidi to control, the sorcerer sealed his creation away in a small prison on Earth before being killed by the Supreme Kai Shin, while Buu lay dormant for millennia before Babidi awakened him in the present day.
Given just how costly and far-reaching Majin Buu's initial rampage was and set so far back in the history of Universe 7 -- before the advent of Super Saiyans and the like -- it was likely Bibidi and Buu who shook the natural evolution of the universe. With hundreds of planets and millions of lives lost as part of this onslaught, plenty of civilizations and races were cut down before they could evolve at the same rate as many of the populaces in the other eleven universes. And the difference in this rate of evolution can be seen right in the neighboring Universe 6.
Given its proximity to Universe 7, Universe 6 also boasts its own race of Saiyans, Namekians and the race of powerful beings like Frieza. Though all three races in Universe 7 survived Majin Buu's initial onslaught, Universe 6's Saiyans and Namekians appeared to be stronger than their neighboring counterparts. This is evident with Universe 6 Saiyans like Kale and Caulifla easily achieving Super Saiyan transformations in their own right, with Caulifla reaching Super Saiyan 2 and Kale reaching Legendary Super Saiyan. In contrast to this, only a select a handful of pure-blooded Saiyans in Universe 7 were able to achieve the Super Saiyan transformation -- and that was only after decades of intense training.
Even with the setbacks Universe 7 and its natural development of mortals has endured in comparison to the other universes, the world has more than proven itself capable of standing up to those widely considered more advanced. Dragon Ball has always been a franchise that celebrates the power of the underdog. The Tournament of Power takes that theme and elevates it on a multiversal scale as Universe 7 proves that mortal power rankings mean little on the way to its ultimate victory.