The Bible uses different words for repentance. In this parable, the son's 'repentance' is a change of mind that leads to action, but it's not the same deep transformation the prophets called for.
In Matthew 21:29, the son says 'I will not,' but then 'he repented and went.' The Greek word used here for 'repented' (metamelomai) often signifies regret or a change of mind, especially concerning a specific action or its consequences. It's different from the more profound word 'metanoeō', which implies a complete heart and life transformation, a turning away from sin that leads to salvation.
Think of it this way: the first son's 'I will not' was raw, immediate disobedience. His subsequent 'repentance' was a realization that not going would have negative consequences or perhaps a simple shift in his feelings about the task. It led him to do the father's will, but the reason for the change is key. It wasn't necessarily a deep spiritual awakening, but a turning from his initial refusal.