Most millionaires do believe in the traditional type of investment- which is either purchasing stocks or gold on the market. Since most of these investors got their wealth from a relatively risky method, some do not approve of investing into an instrument which is highly volatile (cryptocurrencies).
Though this may be the case, some big investors are jumping on the ship as they expressed their interest towards cryptocurrencies. Imagine a millionaire purchasing several hundreds of BTC. The slightest increase/decrease on its price means gain or loss of millions of $$$ on their part.
Either some millionaires hate Bitcoin or they just don't understand how it works. The vast majority of wealthy people are against Bitcoin, because they believe it's a scam or a fraud. Warren Buffet and Donald Trump already expressed their disdain towards Bitcoin. They're missing a huge opportunity for profit as Bitcoin continues to soar towards new ATHs. The fact that Bitcoin is scarce and decentralized, should be more than a strong reason for anyone to invest into it for the long haul.
At least, there are some millionaires putting their money and faith into Bitcoin. Elon Musk and Mark Cuban have joined the crypto/Blockchain craze. Not everyone will find Bitcoin as an attractive investment, because of its widely unpredictable nature. The constant fluctuations in price, makes people think twice before relying on Bitcoin as a long term investment. What matters is that Bitcoin stays decentralized and censorship-resistant. As long as there are loyal supporters behind the project, there should be nothing to worry about. Just my thoughts