These days mics are low impedance mics, just like the mixers etc. you plug them into. But until somewhere in the sixties everthing used to be high impedance.
Guitars and amplifiers still are high impedance to this day. This is why high impedance microphones are used for harmonica, because harmonica players often plug their mic into a guitar amplifier, not a mixer.
Furthermore, a lot (not all) of those high impedance microphones were crystal or ceramic microphones, which cause for a raunchy/distorted sound. It's like the effect of talking through an old telephone, which indeed all had ceramic elements.
The Green Bullet is a microphone that Shure has been making for over 50 years. Specs have chaged a bit of course, but it's still a high impedance mic with a ceramic element for a slightly distorted sound.