The primary purpose of art is as a store of value - i.e., in some ways it behaves like a currency. This store is extremely important to rich people, who can thereby store tens of millions of dollars worth of value in a single canvas. But there is no central regulator of this currency, and no treasury issuing notes - its value comes entirely from perception and some historical myth-making.
Forgery here has the same impact as forgery in any other currency - it debases the value of the currency. But imagine if you could debase an entire currency by issuing a single false note. Art forgery confuses the value of art - is it the object that is important? Apparently not - the only possibility, then, is the chain of authenticity demonstrating, e.g., "Picasso made this shitty painting". Forgery means the painting as object is less meaningful AND it means demonstrating the chain of authenticity is more important to maintaining its value. This is really bad for some rich people who own a lot of art.
Embarrassing art connoisseurs is roughly equivalent with high treason in that world. The last thing they need is for an actual artist to show they're full of it.
Just like the clothiers didn't like the boy in the story of the Emperor.
> The second reason is more nefarious: he was pranking the art world.
More nefarious?