It's easier for a speaker manufacturer to get a steady impedance (which amongst other things increases the potential for frequency linearity) with lower impedances than higher numerical impedances.This statement is false in several ways. Impedance has nothing to do with the linearity of the impedance curve- that has entirely to do with the design of the speaker. Secondly, a linear impedance curve likely benefits a tube design rather than a solid state design- the real issue is whether the amplifier can operate as a true voltage source- if so the linearity of the impedance may well be moot.
A long time ago amplifiers had a great deal of trouble dealing with low impedances and high power. With the advent of reliable ss amplification that can double down, speaker designers were free from those constraints to advance other aspects of speaker design.This statement is also false. What really happened in history is that the idea of an amplifier being able to operate as a voltage source was proposed by EV and MacIntosh in the 1950s. The idea was to eliminate the guesswork of setting up a loudspeaker. At the time, a speaker was usually set up with a midrange control and a tweeter level control. These were not there to adjust the speaker to the room, they were allow one to adjust the speaker to the voltage response of the amplifier used (as such, both the speakers and the amplifiers in use would be what I call Power Paradigm technology).
With the introduction of the Voltage Paradigm, level controls on the speaker were no longer needed.
At the same time, the industry was transitioning from tube power (which is expensive, so much so that most loudspeakers were fairly high efficiency) to solid state. It was a lot cheaper to build a solid state amp (no filament circuit and no output transformers) and yet the industry was able to charge nearly the same money for the amps. Solid state amps, requiring a lot of feedback for linearity, had a side benefit from that feedback (and the otherwise lower output impedance of the output devices) which was that they could much more easily operate as a voltage source.
The significance of this (especially the increased profit) was not lost on the loudspeaker industry. It takes a lot of precision (costs more) to build high efficiency loudspeakers, and so with the new higher powered solid state amps, it was possible to build lower efficiency speakers and yet **charge nearly as much for them** (and if you are following the dollars here then you see what this transition was really all about). To get back some of the loss in efficiency, loudspeakers began to appear that were 4 ohms rather than 8 or 16 (or even 32 ohms...). The lower impedance asked more power of the solid state amps, and so a speaker might be 10x less expensive to build and not really seem to be all that much harder to drive (while costing the customer nearly the same retail dollars). Of course the amplifier ran hotter...
In time solid state amps also became reliable driving these impedances, but to say that there was a constraint removed to advance speaker design is not at all accurate, unless you look at it from a perspective of profit.