They are the ones with the data – there must be certain numbers that come up frequently enough that if you knew them would access an unhealthy number of cards.
I’m going to hazard a guess that there are over 100 million accounts in the UK, and only 9999 possible PIN numbers. Surely any combo is going to give access to an unhealthy number of accounts if that’s the reason.
I’m going with 8008. Bet no has ever thought of that.