Loudness - Why has the industry stopped producing amplifiers with this feature any longer?

I used to have an ARC SP3a that had something called a "Contour" control. Basically, it was a tuneable "loudness" control. I don't recall seeing it on any subsequent models. If the tone controls were activated you could control the curve to add more bottom end for low listening levels. I never used it all that much. Being as this was mumblety years ago, I seldom listened at low levels anyways. But I could see it as a handy tool for apartment/condo dwellers or for background level music. 
Cheers... 

It fell out of favor, when people  were told set there tone control to  neutral and loudness switch off.

Schiit Loki.
This 4 band tone control is marvelous. I use it as a loudness switch for low volume evening listening. For some harsh recordings I tweak the mid-high and high freq controls - but very infrequently. So once set, I just switch it in or out as needed. My Manley Stingray integrated amp allows me to do this with the remote.

I hate people who hate tone controls! Not because I disagree with them, but because they have ruined the audio world for the rest of us. They proclaim Tone Controls = BAD!! Ummm... but there’s this thing they invented call a SWITCH..... and you and just switch them out of the audio path.... and don’t get me started on "oh but that switch is in the signal path and it degrades the sound too..." No! It does’t.
As for the Loudness circuit, there is very sound psychoacoustic reason why this is a fabulous idea! Our hearing loses bass at low volumes. Gee, why can’t we have a circuit that increases bass as we turn the volume down? That would make quiet music infinitely MORE ENJOYABLE. Isn’t that what we want? Music to be more enjoyable?? Well thanks for ruining it for all of us you tone haters! Oh... and Yes, variable loudness is much better than a fixed amount.

So long as you have a variable loudness and variable volume control, this is a very useful control to have.  I'm afraid it was sacrificed to the "less is more" philosophy, along with (in come occassions) balance controls.  Purist, perhaps, but consumer-unfriendly.