It's a fairly long story, but here goes...
First "Fox Grapes" are American grapes. There are several species of vitis that are used for wine production. I think in this case it's vitis labrusca.
So, here's what happened. Somewhere in the 1860s a French researcher brought grapevines from the USA to France in pots with soil. He planted them in a vineyard to study them. What he didn't know is that he brought two problems back with him, powdery mildew and phylloxera.
Phylloxera is a root louse you can barely see it with the naked eye. American grapes, having been exposed to this bug for millions of years, adapted to this bug and is now resistant (resistant to powdery mildew too). European grapes, all which are one species vitis vinifera, had no resistance to this bug. So, in a few short years it decimated almost all European grapes.
Researchers were desperate to figure out a way to combat this bug. There was a three pronged approach. First they poisoned the soil and that works but is bad for the soil. It literally kills everything in the soil. Bugs, animals, fungus. Most of which is beneficial. This was eventually outlawed in the Europe.
Another way was to graft american rootstock to European grapes. This is the way almost all grapes in Europe are grown now. The third way was to breed American vines and European vines. This was quite popular too since it eliminated the hassle of grafting rootstock.
These newly bred vines were called "hybrids". (BTW, Isabella was not one of these newly bred grapes. They think it was a chance encounter between European grapes and American grapes in the USA as people tried to plant European grapes in the USA). Anyway, back to the hybrids. Many, many new varieties of grapes were produced and with careful breeding they could make decent wine but never quite as good as 100% vinifera.
The French, leading the charge, banned all non-French grapes from being grown in France as an act of purity. They force untold acres of grapes to be dug up and replaced with pure French grapes. This spread to other parts of the EU several decades ago and now very rare to find these old hybrids being grown.
There are new efforts at using advanced breeding techniques to breed grapes that are almost indistinguishable from 100% but have hybrid features to combat phylloxera and powdery mildew and reduce the amount of chemical sprayed in the vineyard. Hybrids are still quite popular in the USA for these reasons and also they are quite cold tolerant and can withstand much colder temperatures than what is typically found in Europe.