Because the Brazilians didn't want a European and the Italians didn't want a South American and because in those days there were no Asian or African elite referees, FIFA settled for the referee from the DDR. Rudi Glocker had only a few matches to his name with national teams, but the final of the 1970 World Cup was the beginning of a long and successful career.
Glocker died in 1999.
matches
Glockner(1929-1999) played for Rotation Leipzig and took his referee exams in 1952. He was a DDR-Oberliga referee between 1959 and 1977. He officiated in 4 DDR Cup finals.
Among his 105 (source: FIFA) international matches were one in the European Championships, three of the 1964 Olympic Games, and quite a few finals:
finals
The 1970 World Cup
The 1970 International Cup
The 1971 Fairs Cup
The 1974 European Cup Winners' Cup
The 1976 UEFA Cup
4 DDR Cup finals.
In total he must have taken charge of at least 1150 matches.