I think it's simply a changing of the guard.
You get gifted opportunities to experience, exploit, benefit from etc certain nuances in the particular moment in time. I think it's a sort of 'you have to be there to understand' thing.
What you know from your past is based on your generation, something which doesn't last despite you perhaps believing how things were for you is how it was, and always will be, for everyone else.
And with drugs, I think it's about the sub-cultures that are connected to the use of those drugs. That's where you find the time limited window into specific social and cultural paradigms that define that particular sub-culture. Of those paradigms might be social norms and values that emphasise certain shared cultural beliefs etc. When you look at the dance scene during the eighties and nineties, you can see that sub-cultures had a huge part to play in the exposure to the wider world of certain cultural beliefs and values that then rippled through society and along with those rippled provided the conduit for others to partake in similiar involvement, of which might have been the exposure to really good MDMA as a byproduct of being connected to those sub-cultures. That's why many people got wind of there being good MDMA around those times, because the influence from those sub-cultures was hard to ignore. It was effectively what the rave scene, hard dance, techno scenes etc were all about. You couldn't really put the eighties/nineties, hard dance and MDMA apart from one another. This is very much a cultural thing and along with it were the nuanced cultural values attached. And that often was having a really good time, which involved really good MDMA.
In past generations, you had more access to sub-cultures because the world was more divided, at least in the sense that localized cultural identity existed. There was a lot less pressures on local culture because the world was a lot smaller. There was more depth to local culture which allowed for sub-cultures to flourish because the overarching culture was isolated from the rest of the world, therefore more likely to be preserved. Today we have social media and the homogenization of culture until we are all effectively one big gloop, instead of pockets of islands with our own ways of life. And it's in these pockets of islands where certain nuances can be found, certain time limited windows into a particular way of life and along with it, particular perks of being there at that time. When one mainstream culture dominates all, the rest are left straggling under the pressure to conform to the overarching norms and values.
Naturally time moves on and due to rapidly changing world and the way in which we live our lives socially, what was once the norm and essential to way of life, is now secondary and obsolete and fading into history. I can definetly see that over the last few decades there has been a huge change in the quality of drugs available and I think it's heavily linked to the changing of culture and how people perceive what is important about them. It's more about quantity than quality. It's more about the superficial process of consuming them, as apposed to their usage having beneficial and meaningful purposes ie used in connection to a particular setting, for a specific purpose, with particular values and beliefs attached etc.
For good drugs to be central to peoples lives, people have to first believe that is important and secondly have been fortunate enough to have experienced both good drugs and a sense of identity to a particular set of cultural beliefs that puts the importance of this as a top priority. Today people would rather score impure, perhaps even bunk, MDMA because culturally they have no sense of direction when it comes to identifying with what they are doing, no rituals, no community, no wisdom nor advice garnered from past generations, no connection to a particular shared reality that considers this important.