Because instead of washing with water alone, when it is scraped with wood and washed with water and lotus, it is more effective and efficient in removing the trace of blood.

Sayyidah Umm Qais bint Mihsan (may Allah be pleased with her) narrates that I asked the Messenger of Allah (peace be upon him) about menstrual blood that gets on clothes, so he (peace be upon him) said: "Wash it with water and lotus leaves, and scrape it off with a piece of wood."