MSE News: Monzo introduces overseas ATM fees

My belief is that Monzo had no particular intention to be regarded as a 'travel' card at first. Until late 2016 there was no visible mention on their website about being fee-free overseas, and I had to ask on their chat about fees and exchange rates. Soon after, they began publicising their usefulness as fee-free for overseas travel.

So it's likely that they never originally factored in the substantial cost of all the users who subsequently made lots of fee-free overseas ATM withdrawals. Their only revenue, the interchange fees etc when the card is used for purchases, was simply not enough to cover the unexpected cost and survive, without introducing the foreign fee-free withdrawal limit.