... Now here is a similar situation today: Retro/D-3 racing: scratchbuilt chassis built by the race winners, then these same folk selling their hand-built chassis to others for joining into this class... again, raceways NOT selling chassis and/or RTR cars to would-be racers...
Don't get me wrong here - I am one to be joining into retro and D-3 racing very soon, as well as true vintage 1960s racing. I just happened to notice this similarity in your posts, and thought it might be "food for thought". The retro racing, using RTR or chassis kits sold by the raceways, gets $$$ in their pockets on top of the track time and race fees.
And another 1960s reason for shops closing you others brought up: new rules, causing older products to be non-sellers for the raceway owners - I have seen this first hand, today, at a raceway I know of... where constant rules changes got an owner stuck with hundreds of $$ of products now deemed illegal in retro racing... how does that help an owner to stay in business?? I think the racers themselves, as another posted, can be part of the reason for decline in the hobby. We must take the raceway owners into consideration - no raceways, no hobby, unless it all retreats to garages and basements...

Another segment of the hobby that is growing each and every month worldwide: 1/32 scale plastic cars. Like them or not, it is GROWING and 1/24 commercial is... not...(?)... at least not at the same rate 1/32 scale is. Again, food for thought and discussion.