We put this question to Shaykh ‘Abd-Allah ibn Jibreen (may Allah preserve him), who answered as follows:
The ruling has to do with the najaasah itself. Once the najaasah is removed from the clothes, they become taahir (pure). Shaykh al-Islam Ibn Taymiyah said that there is no specific number of washings required for najaasah to be removed.
And Allah knows best.