I'm not a native speaker. If i add a "of", ("because of short attention spans!") that sounds if it was correct in the past aswell, so I'm confused. For me that seems just leaving out the "of", or is there more?
"Not many people read the whole thing because HN."
Now, you could add the "of" and just leave it at that, and take it literally. But the implication is so much more than just saying "because of HN."
"People are so fat here because 'merica."
As I said in another comment, to me it implies the subject is dumb or hasty or something. As in a little bit of caveman speak maybe ("He hit own head because stupid").
> "It means something like 'I'm so busy being totally absorbed by X that I don’t need to explain further, and you should know about this because it's a completely valid incredibly important thing to be doing'"
Because brevity and wit.
I wouldn't worry too much - this is just a fad.
People get bored and do weird things.