I tell you exactly why.
 
Firstly if bass sounded any good it was generally a real BASS and nothing else back then, most bass samples sounded sh1te.
 
Working on Fairlights and S1000's etc the predefined supplied samples were not bass heavy. You might as well ask why orchestras don't have enough bass.... The answer is - it's just the instruments  supplied...
 
Finally we liked the sound of it in the upper ranges, it was a popular sound.  That was the fashion... I remember many producers I worked with openly stated it. It was the trend, and yes it sounded clearer. I think this was the main reason.

Oh I forgot about the delivery mechanism...

Most people had one of these:

 
One of these:
 

 
Or one of these:
 
 
 
The only woofers were dogs.

jbow

batsbrew
early albums were mastered for vinyl.
 
if you play vinyl too loud, the needle will jump out of the grooves.
 

We used to play vinyl pretty loud with those big 70s Pioneer and Sansui systems. They would get about as loud as you wanted.

It's not about how loud you play it, it's about how wide they cut the grooves in the master and the mechanical demands those high-amplitude, low-frequency signals place on the needle and cartridge. And it might be less about the needle actually jumping out of the groove, and more about the distortion that results at higher frequencies when the low-frequency amplitudes are high and added wear and tear on both the vinyl and the needle.
 
If modern vinyl is being mastered with more bass, it might be because they know that's what everyone is used to hearing in the digital age, and they can get away with sacrificing some purity in the higher frequencies to deliver that thump, especially given how MP3s and online streaming have dumbed down the average listener's sensitivity to distortion. 
 
This is all pure speculation, you understand. I don't know that much about vinyl, and didn't stop to Google. 
 

SO then I was partially right!
Actually, I can vouch for this. I decided while doing some chores to make a playlist of both Birth of the Cool (1957)  and Re-birth of the Cool (1992). (of course re-birth is a re-recording of the original album)
Honestly, there is SO much happening in the chord progression that I NEVER heard (because of the bass), and probably would have never heard without having to destroy the sound quality of the originals. MAN, some of these tunes are even MORE complex than I thought!

OK, I think my question got lost in the shuffle a bit.   

Forget loudness - I understand things got louder... but the spectrum of frequencies has changed.   

Pop a dance track from 1981 into Sonar and then a dance track from 2015 into Sonar.   Look at the graphical EQ plot of the readout.
 
The songs from the 80s have much less bass compared to the other frequencies.   Why did people start adding more BASS?

Was it that they couldn't for some reason?   Was it just not preferred?

Plus almost every decently mixed and mastered album of yore (and IMHO they should still be today) was high passed at 30 to 40hz. This alone does wonders to clean up a dense muddy mix. But today it's Katy Bar The Door. Or don't.

still leads me to wonder WHY more bass in the past 20 yrs or so.   volume I get... but why more bass?   was it a technical restriction?    Like i said, there are bass-heavy current records still being released on vinyl, so i dont understand that as an answer - although i have heard that before.

Doktor Avalanche
I tell you exactly why.
 
Firstly if bass sounded any good it was generally a real BASS and nothing else back then, most bass samples sounded sh1te.
 
Working on Fairlights and S1000's etc the predefined supplied samples were not bass heavy. You might as well ask why orchestras don't have enough bass.... The answer is - it's just the instruments  supplied...
 
Finally we liked the sound of it in the upper ranges, it was a popular sound.  That was the fashion... I remember many producers I worked with openly stated it. It was the trend, and yes it sounded clearer. I think this was the main reason.

dude - listen to some synth sounds from say the ARP2600, Pro One, or Mini Moog... amazing bass sounds...  rich and creamy... so i dont think thats it.

Doktor Avalanche
I tell you exactly why.
 
Firstly if bass sounded any good it was generally a real BASS and nothing else back then, most bass samples sounded sh1te.
 
Working on Fairlights and S1000's etc the predefined supplied samples were not bass heavy. You might as well ask why orchestras don't have enough bass.... The answer is - it's just the instruments  supplied...
 
Finally we liked the sound of it in the upper ranges, it was a popular sound.  That was the fashion... I remember many producers I worked with openly stated it. It was the trend, and yes it sounded clearer. I think this was the main reason.

alewgro
dude - listen to some synth sounds from say the ARP2600, Pro One, or Mini Moog... amazing bass sounds...  rich and creamy... so i dont think thats it.
 

 
If you listen to records with those synths they generally aren't tinny or the producer has turned down the volume (because as I say it was the trend). Those synths were very expensive at the time, not every band could afford one of those. Not everybody used these sort of keyboards and not everybody was using the Human League/Ultravox/moog basslines... Listen to those sorts of records - they ain't tinny.

I'm mainly talking about samplers mainly which everybody used. FM synthesis was everywhere as well (DX 7's/D50's/Fairlights/S1000's)... Well at least during my period from '86 onwards. 
 
But of course what would I know because I was working on these sorts of records in major recording studios at the time if you don't mind me beating my own drum . Sorry ;).

Oh and I forget we generally would have vinyl and a CD master (CD's were just coming in).... they would often sound quite different. There was a time when we would do DAT masters (that was a time best forgotten).
 
right ... Now off to align the tape machine...

One word: Reggae. Those old reggae mixes were plenty bass heavy and pressed onto vinyl too. Reggae, Lover's Rock and Ska were huge in the UK in the late 70's and early 80's and I remember hearing the heavy bass thumping out of pubs and clubs when I was a kid.

Another word: cocaine. Drug trends have always had a huge influence on the music scene and the 80's was the decade of heavy blow abuse. I've heard more than one engineer say that 80's mixes were so brash and top heavy because everyone in the studio was coked up and all that treble sounded awesome....