It's not "inexplicable". uniQure is a going concern with treatments in the pipeline for Huntingtons, hemophilia, and congestive heart failure. They have a finite amount of resources and are allocating them to projects that they're more optimistic about than this one. Glybera is of questionable efficacy and is very expensive to administer (they're on the hook for long-term monitoring for everyone they dose).
It's not at all hard to see why they'd stop production.
How much does it cost to manufacture the drug, and in such relatively low quantities? The article did not say but it pointed to this as part of their decision.
Maybe caving in and lowering the revenue would encourage future customers for other expensive treatments to boycott high prices and hold off for price drops. Refusing to negotiate and forgoing revenue now could be the rational strategy in the long term.
It's not at all hard to see why they'd stop production.