My guess would be the size of the collision space. Bank account numbers tend to be shorter, and depending on date of opening and institution, may have been sequentially assigned. It would be much more difficult to accidentally enter someone else's credit card number than someone else's bank account number. In addition, if you accidentally sent $5k to 61-019341 instead of 61-018341, chances are the recipient would be less than willing to be a good citizen and admit it was an erroneous deposit.