12 Bar Blues was invented by the famous educator B.F. Skinner. He used the chord progression 1-4-5 as a means of teaching math. 4 bars of 1 equals 4. 1 and 4 makes 5.
Why 12 bars? Because B.F. Skinner also loved doughnuts which are sold by the dozen.
He must have had some cop blood in him. There's another version of that story. Mr. Skinner went bar hopping one night. After one last beer in his 12th bar he staggered home, grabbed his guitar and started playing. The only chords he could remember were the I, IV and V, which he played repeatedly until he puked on his shoes. He resumed playing and singing, and called it the 12 bar blues.