When I was shopping for drums, in the 1960's, I remember seeing Sonor kits settup and on display, in at least two music stores. There wouldn't have been too many shops, either. I did not see any Trixon kits, John Grey,Premier or Ajax------to name a group of drums, that would have been considered, to the North American mind, as in a similar league. All of those had to be looked at in catalogues. I eventually ordered Trixon from a catalogue. This was in Toronto, which at the time, was a much smaller place. It does seem like, Sonor had a more vibrant distrubution , at least in this part of the world. That wasn't the case in England, where Trixon was the largest selling imported drum, probably up until 1968 , at least.
The kits are out there alright. My guess, as to why, they are showing up in any larger quantities, on the aftermarket?
When the economy is slow and money is tight, people try to sell , something of reasonable value, that they have been holding on to. A lot of these kits are either, alternate kits of a drummer, or kits that have been in a family --- a son's/daughters kit, a deceased father's kit. The supply will drop off though.