Because the Prophet ( صلی اللہ علیہ وسلم ) said that there is no zakat in silver less than five awqiyah.

Narrated Khalid bin Aslam: We went out with 'Abdullah bin 'Umar and bedouin said (to 'Abdullah), "Tell me about Allah's saying: "And those who hoard up gold and silver (Al-Kanz - money, gold, silver etc., the Zakat of which has not been paid) and spend it out in the Way of Allah (V.9:34)." Ibn 'Umar said, "Whoever hoarded them and did not pay the Zakat thereof, then woe to him. But these holy Verses were revealed before the Verses of Zakat. So when the Verses of Zakat were revealed Allah made Zakat a purifier of the property."