One of the many reasons on why Warren Buffet is against cryptocurrencies is due to its inherent and store of value. He believes that bitcoin does not possess any physical value compared to money, gold bars, real estate, lands, etc.
Another thing, bitcoin (to be specific) has a high volatile value compared to the ones that I mentioned above. Successful investors like him rely on the conventional way of investing with relatively minimal risks to consider.

Although there may be reasons on why he doubts cryptocurrencies, we should not rely his decisions solely.