The short answer? Risk management.
“ACH transactions sometimes take longer to settle as a risk management measure. Credit and debit card transactions are authorized and approved immediately, so unless the consumer later says the transaction was fraudulent, the transaction will typically go through. However, ACH transactions aren’t authorized right away and can be rejected or reversed after they’re submitted to the receiving bank (the customer’s bank).”
Read more on this here: https://healpay.com/2015/09/28/why-do-ach-payments-take-so-long-to-settle/