First off while marijuana has been cultivated for thousands of years it is not as if the plant is fully understood even today so it is not as if anyone could have been expected to breed The Holy Grail at any time unless it was at least partially by luck/accident.
Then consider how when you breed you need a stable of genetics. For the thousands of years marijuana has been cultivated how long has there been any sort of organized distribution system for genetics from around the world? For most of the time period you talked about the most anyone could do it work with what grew wild regionally that they would have access too. Depending where someone lived in the world that could mean all sativa or all indica, and maybe only one or two strains within the size area someone might travel back then. That doesnt give you a lot to work with when it comes to breeding.
It was asked why all the low-grade genetics? The answer is basically stupidity combined with budgetary constraints and greed.
The average grower of recreational strains is always looking for Mr. Toads Wild Ride and every flavor of the month strain that hits the marketplace catches the eye of that type of buyer. Some little clown shoe who never smoked anything better than Roadside Red purchases their first professionally created genetics, they have a little luck and their grow is at least fairly successful, they smoke it and it knocks their bubblebummer-socks off and then they post on sites like this how its the best strain in the entire world. Every other little clown shoe-bubblegummer who reads the message orders then orders the crap, is impressed and the word spreads how the mid-grade genetics are SPECTACULAR
when they are not. Thats the stupidity part.
The budgetary part is many growers cannot shell out large sums of money for top genetics so there is a marketplace for less expensive genetics and less expensive will always mean lower quality. But there are a lot of people who can only afford the lesser priced genetics so a new fresh supply of so-so genetics stream out yearly and the combination of the stupidity factor and the budgetary factor can combine for a lot of sales for differing reasons.
The greed part is on the part of some breeders. Some breeders will not release everything they create because all are not success stories but others will release everything they make. They attempt to market it well, give it a catch name, somewhat embellish how good it is and then sit back and let the stupidity and budgetary factors kick in and when that happens the dollars pour in. Some breeders would rather sit in the evening and look at their investment portfolio than a Cannabis Cup on the wall and those guys will flood the market with just about every single combination of genetics they can put together to keep the bucks flowing in regardless of most of it not being anything special.
Then there is a small segment of the market that only wants lower to mid-level genetics. They are ones that enjoy an easy going light buzz where they can fully function as they want to but with that little extra chipper feeling going on in the background. As difficult as it might be for some to believe, that is all that some people want from what they smoke. So there is a legitimate marketplace, small though it might be, for middle of the road genetics.
When it comes to what some med strain smokers say, take it with a grain of salt. In some cases what some say is true about quality but for some reason in some med growers/smokers a curious psychological phenomenon occurs where once they are legal and they have access to med strains, even if they are not really better they instantly become by far the best in the entire world. Why? Because that person can get it and most others cannot.
In other words, some embellish.
I have a friend who lives in Northern California who has a med card and he bragged up a storm about the worlds greatest strains were not only med strains but ones found in Cal. I did have to admit a few I have tried were very good, but not way better than other top genetics, and some he said were fantastic were not as good as things I smoked in the late 60s and 70s.
Some are very high quality, but others are not nearly what they are made out to be by some.
What I find curious is how med strains are expected to be Mr. Toads Wild Ride when they are not bred for recreational purposes and instead for medicinal purposes. If someone suffers pain or a eating disorder or something their brain does not need to be in Oz to get the type of relief they need.
If people breeding medications believes that they have to put the patient in the ozone when they smoke it along with actually treating the patients medical problems then someone else needs to be breeding medications.