[Someone]

The honest answer probably would be “Now that you left the EU, we thought ‘let’s see what we can get away with now’”. https://en.wikipedia.org/wiki/Interchange_fee#European_Union:

“In March 2015, the European Parliament voted to cap interchange fees to 0.3% for credit cards and to 0.2% for debit cards, which was subsequently enacted under Regulation (EU) 2015/751 with effect from 8 June 2015. The caps apply only to personal cards where there is an intermediary, not to cards issued to businesses or to cards issued by American Express.”

[llimos]

The need for a legislative cap is just as much an indication that (to quote the article) "the market is not working well", as the rise itself in the absence of the cap.

Why not work to fix the market instead? Or nationalise it, since there's really nothing for intermediaries to compete on any more.