Why are grape preserves so hard to find in the USA?
TL;DR: likely because demand was too low to sustain commercial distribution

Barring any relationship to a specific historical incident, it's very difficult to find out why a specific food is no longer commercially produced. In the absence of specific reasons, we can see a number of contributing factors as to why grape preserves would have gone off the market:


In the US, sales of preserves in general trail sales of jams and jellies considerably.  Grape jelly eclipses sales of grapes in other forms (sorry all refs behind paywalls).  This distribution is very skewed with 9 varieties constituting 80% of the market.  So it could be quite possible for grape preserves to "fall off" the bottom of the market.
For historical reasons having to do with the Concord Grape, Americans are used to eating their grapes as jelly, so preserves may seem quite oddball and hard to sell.
From what I could gather from the one grape preserve recipe I found, the runny texture of grape preserves are possibly off-putting to most consumers.


Not terribly satisfying, I know, but that may be as much information as is out there.