Because we can't use bitcoin in our daily living. Imagine when you are hungry and you need food asap.
Well that is the only the drawback towards Bitcoin, but that alone is not enough to be a reason to make cash better than Bitcoin, I mean it already have reached higher than cash, if you could see it obviously, its not 1 btc = 1 dollar (or any currency). That is only value wise, and for the fact that we don't need have tax in order even if you earn so much from this community, so its a win win. Just cash out a monthly budget for God's sake.