They are required by the government to implement strict anti-money laundering systems and closely monitor traders' transactions. In an interview with Bloomberg, Discover CEO David Nelam said that for the financial institutions to process the payment for cryptocurrency traders and investors, this debit card is to buy or sell bitcoin or any other cryptocurrency in the market. It is not allowed to use its Discover credit on the Cryptocurrency Exchange and merchants' brokers.
Source - https://www.ccn.com/discover-ceo-explains-why-credit-cardholders-cant-buy-bitcoin/