The question is really "why wasn't it?"... XUL, conceptually speaking, had a lot of potential, largely squandered by Mozilla-the-corporation. Somewhere between the implementation based on RDF (a literal plague over web tech) and confused commercial strategies, it died a slow death.

I call it "The Law of Conservation of Unusability". This states that whenever a technology gets to the point where it becomes usable, where you become comfortable with it and are happy with it, something will always happen that either makes it completely unusable or makes it significantly more difficult to work with. This "something" often has no reasonable justification and the reasons for it look quite artificial.

Also known as “why we can’t have nice things”

Most of the time this does have a reason, and I call that reason "the consultant effect", an effect having a slight resemblance to the phenomenon from the Upton Sinclair quote, "It is difficult to get a man to understand something, when his salary depends upon his not understanding it".

In this instance, though, the reason is just the organizational incompetence that Mozilla is now known for.

It's amazing that for the past 8 years (almost to the day!) imgur has been hosting your screenshots.