There are multiple potential explanations. I think disallowing commerce without an agreeable substitute, like immediately transferring or escrowing money from other accounts, is an unacceptable inconvenience. Often times, a person wants the protection of a credit card, while having sufficient assets to purchase in cash or repay the card corp. If anything, that a debit card lacks similar protections of a credit card is absurd and inconvenient per skimming and missing fringe benefits like travel insurance.

Debit and credit are the only methods for commerce with almost any third party. Transferring and escrow payments require direct channels that Wells Fargo does not want to negotiate. Regardless of differences between debit and credit, debit is at least backed by direct account balances. Credit is not. Chargebacks would also be a nightmare for credit cards.

Even if it's deemed unacceptable to disallow over-risked services like credit cards to purchase cryptocurrency, then the beauty is that cryptocurrency purchasers can migrate their loans, and savings to another banking provider who will service the headache of credit-backed cryptocurrency investments.

Banks are in the business of risk management. They apply that to money, but ultimately, they are all about that risk.