Yes, the idea of forcing all financial providers to support API Open Banking is being looked at by regulators, but there is concern that it would be too costly for some small providers so they are reticent to force the issue.
The idea, I think, is that everybody else already supporting it should act as a kind of soft pressure on competitors who would be forced to implement support so they could catch up, or risk losing customers. It doesn’t seem to have worked so far, but maybe John Lewis could be persuaded down the line?