Because bitcoin is not a currency that goes according to book or guild-lines.Warren Buffet always reading everything that related to company, that he hoped to invest in company.He said he read 1st annual report of Coca-cola before investing in late 1980s. He is hoping small growing speed and low volatile rate.Normally 0.1%-15% max.But the bitcoin has highest growth rate of any stock.0.001 USD to 20K USDs just in decade of time.This high volatile is not the style of Warren Buffet.Its break of the stocks laws.So I think that is why he don't like bitcoin.