Why are there speakers designed with widely fluctuating impedances and steep phase angles in the first place? Why is there not more of an effort to build speakers with flatter impedance curves and gentler phase angles?
IMO, it is because building a speaker with a flat impedance & phase curves requires some serious engineering & knowledge of physics. Not every speaker manuf has that; in fact, most do not. They cover this up with marketing hype - computer-aided design, CNC manuf, lustrous finish, exotic woods, Berrylium & Diamond drivers, blah, blah.
There are several trade-offs in manuf speakers such as on-axis & off-axis response, power handling, sensitivity, resonances, tuning the port, bass overhang, etc, etc. I am no speaker designer expert but I've spoken with a few manuf in the many years I've indulged in this hobby.
Most of these speaker manuf will make certain trade-offs that will yield wild/roller-coaster impedance & phase curves. With the wide availability of class-AB high power power amps, they figure that some amp on the market will drive their speaker. And, coupled with this is the reliance by the speaker manuf that many, many listeners will not know the difference & not know any better. A speaker that is distorting due to impedance & phase mis-management can be sold to these people as one having more "details" in the sound. Also, such speakers have a "wow" factor - hear a demo in the audio store or in-factory. The person is sold & buys the speaker. A little while later you read a "listener's fatigue" post on Audiogon from this person!
This is gradually changing & will force the speaker manuf to wise-up & not output junk designs but the process is slow.
That is why I hear: ARC used to voice their amps with Vandersteen, Avalon with Spectral & MIT cables, B&W voiced with Classe amps & so on. Deviate from this combination & you will be in sonic hell (as Bifwynne! :-)
I'm sure that there are other (better) reasons & I hope that some other more knowledgeable members chime in.