Film processing for negative film is far more forgiving than E-6 process.
Home processing of C-41 films will easily yield negatives that will print well.
Home processing of E-6 film is a dog's dinner of variability starting with the chemistry available at retail.
I cannot tell you how many people would come into the lab and ask what was happening with their home slide processing. Their color varied wildly despite their pride in temperature control and processing technique.
You got a filter effect here. Of course those who couldn't process E-6 properly asked for advice. Those who could, didn't. Not everybody has the same DIY skills and knowledge (despite what they tell). Frankly, knowing myself, I wouldn't attempt to develop it without a processor.
Do you mean Kodak, Fuji and others were selling bad chemicals? And "retail shop" means little - there are large retail shop from which many pros buy from too, and others who have just non-pro customers. You got very different results from labs too, and some labs were inconsistent too (especially for consumer processing), probably depending on what side of the bed the technicians got up that morning... or if you were lucky your films were processed when chemicals were still good enough.
As a student I was convinced of the superiority of transparency over negative film. However, once I learned to print Type-R, Cibachrome and Type-C prints I saw the inhernet superiority of negative film over transparency in its ability to render contrasty scenes well.
If one was truly superior, the other would have had no market. Both had their inherent advantages and disadvantages. For example negatives were harder to scan, so the press industry preferred transparencies until digital. Some photographers too for their own reasons.
For all the praise of transparency film for its "accuracy" those same people then enthuse about its deep saturation that has only a passing acquaintance with accuracy.
Not everybody was looking for "scientific accuracy" - and after all negatives didn't had that either, just the print stage allows for changes. For many "accuracy" meant you got consistent results from the same batch of film. Thereby you could test and know what you would have got.
Anyway Franco Fontana used to duplicate his Kodachromes directly onto another Kodachrome to reach the saturation his photographic style required. If you are a commercial photographer in other sectors, you have different requirements.
In the end, when talk about using film today we are talking about paying a lot of money to achieve a result that emulates a random JPG file with color, contrast and crossover issues created by the process that is largely uncontrollable by the photographer.
Oh well, Lomography build a whole business around it. But it is not true the process is "largely uncontrollable".