Raditz's aptitude was not that poor; he simply had never received systematic training since childhood.
The Saiyans' training methods were simple and brutal. They judged a child's potential and rank based on their Combat Power at birth, assigned them to teams, then threw them onto suitable battlefields and let them grow through combat.
His Combat Power had been decent as a child, so he had been considered one of the promising ones. That was precisely why Raditz, despite his ordinary background, had been assigned to the same team as Nappa, a descendant of a prestigious family, and had luckily escaped the disaster of Planet Vegeta's destruction.
This growth-through-battle model was actually rather awkward for those whose Combat Power was neither high nor low.
Those with low Combat Power had already died in countless battles.
Those with high Combat Power could face enemies of similar or greater strength and gain real combat experience—if the stronger one could not win, the other Saiyans on the team basically had no chance of victory either; if he could win, the "little brothers" on the team basically had no opportunity to face powerful enemies.
What progress could come from endlessly slaughtering weaklings? This was not a game where one could accumulate experience and level up. It was hardly surprising that Raditz had made little progress in that environment.
It could be said that Raditz's best years had been wasted. Now that he had come into contact with Earth's training methods, he had to start from the very basics. Although he had learned a few things from Son Goku, he simply could not understand concepts like "feel the ki within your body," leaving him utterly confused.
As a result, he ended up receiving the same treatment as his nephew: listening to Piccolo's lessons!
Piccolo's teaching methods