One possible reason is that ACH payments are MUCH more user friendly if they can be initiated by an end-user authorizing their bank account (compared to digging up their account #, routing #, etc and entering the info manually).

ACH payments are essentially free to process (or a very small flat fee). This is very different from credit card transactions that charge a % of the entire transaction.

If ACH / direct payments from bank accounts became more common through services like Plaid and Stripe's new service, it could mean less fees (less revenue) for Stripe to collect. Which could explain why it's not something Stripe jumped into earlier.

TLDR: if I had to guess, there's more money in processing credit card payments, and much less money in facilitating ACH transactions.