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

Like Vermonster, I have sold the Yamaha variable loudness concept for years, and he explains it perfectly.  [They actually did make one stereo receiver model with VL on the remote (it had 2 motorized pots), as Jovian notes...this shoulda been the default design].  But I want to mention the approach taken by Crown Int'l with their IC-150 preamp.  Like most pushbutton loudness buttons, it grossly overcompensated, making it a glorified bass boost at all levels...BUT...in the manual they explained its proper use...with the VC set to 2PM (where its effect disappeared), the user was to lower the input gain controls on the amplifier (which theirs had) to set the inflection point where the ear heard full range...thus calibrating the loudness button properly, making it much more effective.  Also, McIntosh has a variable loudness knob on their C-20 classic tube preamp...and it's the best one I've found, with a well judged contour and that's based on 1960s research on frequency dependant hearing perception.

"...with a well judged contour..."

Wow. Now I remember that some brands had "contour" switch.

I think Grundig was one of them, but am not sure anymore.

I haven’t owned a single piece of gear for either home or car audio that doesn’t have a "loudness" feature (it is sometime cleverly renamed) ; and I never will. A true variable Fletcher-Munson is where its at. I like the fatness/punch and warmth it provides at low volumes.
And for what it’s worth - a lot of "purist" are not really purists anyway. How many are running Dirac, or Audyssey, or ARC ? Shouldn’t they be physical treating their room rather than using processing to flatten their response? Room correction processing is just the new/hip thing - so they are ok I guess.... I say use it all - what ever gets you to the experience that puts a grin on your face and that warm fuzzy feeling from head to toe when your system is singing just the way you like it......

I use the loudness on a McIntosh tube tuner, for me it gives the music depth and body, fills out the sound, something is missing without it. 

There are some preamps out there with bass & treble adjustments on the remote. Emotiva makes one.
My old Luxman preamp had both a loudness button and bass & treble controls.
I end up doing a lot of low-volume listening with my Maggies....not a good combination...but bass & treble controls make it enjoyable. 
Have you considered a Schitt Loki equalizer?
https://www.schiit.com/products/loki