Because as a financial expert, Bitcoin isn't tangible. It's not a "real" asset to him. It's only backed by the Blockchain, which he has stated that he loves, by the way:
The blockchain is a technology which is a good technology. We actually use it. It will be useful in a lot of different things. God bless the blockchain.
But it's not only him. Any other financial expert, analyst, or trader will tell you that Bitcoin is a scam/fraud and that the bubble will burst one day. And that Bitcoin will crash.
The only value of bitcoin is what the other guy'll pay for it. Honestly I think there's a good chance a lot of the buyers out there are out there jazzing it up every day so that maybe you'll buy it too, and take them out.
He's probably referencing instances in his personal history where he's seen situations like this happen, such as in penny stocks and pump-and-dumps. He truly believes that Bitcoin is fraudulent, and he's adamant that he "doesn't care about Bitcoin":
...this is the last time I'm ever going to answer questions about bitcoin because I really don't care...
I don't personally understand the value of something that has no actual value. You all can do whatever you want and I don't care.
I could care less about bitcoin. I don't know why I said anything about it.
Every day, you have CNBC, nonstop bitcoin — Who cares about bitcoin?
In my personal opinion, based on everything I've heard him say about Bitcoin, he just sounds bitter. He keeps claiming that he doesn't care about Bitcoin, yet he keeps talking about it.
Look, everyone has a personal opinion about Bitcoin. I remain highly skeptical of it. But as I’ve said previously, I’m open-minded to uses of cryptocurrencies if properly controlled and regulated
I think that he missed the boat and deep down inside, he's wishing he would have invested in Bitcoin when he first heard about it. But what do I know? I'm no financial expert