The Forgotten

The Richard Brown Brown Electronic Lab BEL 1001. A superb 50wpc Class A amp from the 80's. I have the first production one made - serial number 0001.

LAB 12 is great gear .   I have their DAC 1 Reference and it sounds really good.   Built like a tank and 5 year warranty is pretty good too.

A lot of the smaller companies end up going out of business as owners age or pass.  Other stuff just falls out of favor due to lack of publicity.  A random list of largely forgotten gems once upon a time that we don't hear much about anymore:

B&K Electronics

Avalon Acoustics

Merlin speakers

Discrete Technology

Grant Fidelity

Melos

Audible Illusions

Lazarus

Well Temperered

I'm always surprised that Janszen speakers don't get more attention, given the compact size, ease of placement, reasonable prices, and great sound (low distortion, even FR, great transparency, superb imaging). Perhaps the issue is that Janszen doesn't release new models every year or two.

Totem, Avalon, Audible Illusions,  and Well Tempered all still have major consumer profiles. In fact, totem is the highest volume Canadian speaker manufacturer. 

Many audio companies come and go based on consumer tastes and finances. Others, like @knotscott pointed out, dissipate once their owners/designers leave. Blue Circle comes to mind, as does Clear Day and, more recently, Herron now that Keith is retired. Or Merlin speakers once their owner died in 2015. 

Others, like Halcro or Cairn, were badly conceived and faded after the initial advertising blitz. 

And I think many audio companies are similar to music artists like Mindy Smith, Cassandra Wilson, or Pet Shop Boys: After an initial splash, they build a steady career with a steady audience. Think Acoustic Zen, Decware, PSB, Canary, and Channel Island.