@speakermaster,
It has been postulated by others on this forum in the past that the high impedance speaker's decreased manufacturing was due to,
1 The increasing availability in the 1960s of transistor amplifiers that made obtaining more watts much less expensive compared to tube power.
2 A good quality high impedance speaker is relatively more expensive to build and requires more effort. These 2 factors are said to have led the shift to 4 ohm speakers becoming more popular and the default choice.
Charles