I think all famous coin is safe for investment, as long you invest on the right place or hold it on the great wallet and that is safe. But why most peoples say if Bitcoin is safest coin for investment? I think this because Bitcoin as the king of crypto, which Bitcoin influence the other cryptos price, and Bitcoin have many users or investors behind it and supported by little bit of coin supply.
Because of that this coin have great prospect and promising in the future, and many peoples interesting with Bitcoin.