You can currently buy 16 ohm speakers off the shelf from a variety of sources (speakers for guitar amps, hi-fi, and automotive) and historically speakers were made in a VERY VERY wide variety of impedances, with some going as high as several hundred ohms.
Regarding SS amps: none of that mattered when everything was transformer coupled. You just got the transformer wound for the speaker you had. When speakers were almost only sold in big integrated boxes (you know the old style tube radios) this was not difficult or even prohibitive.
Regarding why we dont have other impedances TODAY I'l ask you a question, and tell you the answer.
Are 4 and 8ohms better, or is that just what non-technical users are familiar and comfortable with? Its the later. The best item for the job depends on the job. Period. Anyone who gives you a default answer on BEST is a fool. People default to 4&8ohms because that is what they are familiar with.
I would argue that 8 ohm speaker are standard because most audio power amplifiers are optimized to work best into 8 ohm speakers.
BTW, this has nothing to do with impedance matching or Maximum Power Transfer.