He developed an awesome reputation as a safe cracker by knowing the factory default combination, because nobody ever changed it.
Most of the time, the answer is right in front of you. It is human nature to overcomplicate things and so think of complex solutions first when trying to solve a problem.
When trying to guess passwords, the fact is that most people rely on simple passwords. At my company, you had to change network passwords every three months and they had to be eight characters long with at least one letter, one number and one special character.
You would not believe how many people thought they were clever by using the season, an underscore and their four digit employee ID. So many people would cycle through these four passwords as each three months came up (with "1234" being their employee number):
Winter_1234; Spring_1234; Summer_1234: Autumn_1234
When I worked at Logan Airport in the 1980s, before HID cards came into being, the doors leading out to the ramp area (where the planes were) had a keypad where you had to punch in a code. The number was almost always penciled in on a nearby wall, on the inside of the door frame, etc.
Even to this day, people still tape their spare car key in the wheelwells of their cars and have a spare house key taped to bottom of mailbox, etc.
It's a good thing most criminals are stupid and have no common sense.
...and sometimes as simple as looking in the desk drawer.