My question got closed as too broad. It seems to me that the question could be answered concretely--either such an application exists or it does not. Why is it too broad?
I would like to know so that I do not make the same mistake again.
Well, it's got a couple repoen votes on it, 7 upvotes, and a bunch of good answers, so... you probably shouldn't sweat its closure too much.
Having said that, the reason it's too broad can be found in the opening description of the close reason:
There are either too many possible answers,
Basically, most sysadmins could name multiple systems they've had to scale up, so a question asking about the systems they've had to scale up invites everyone to give examples, resulting in a multitude of equally valid answers.
Going the other way and asking simply if something exists or not, and inviting a simple "yes" or "no" answer isn't very useful either.
That said, you could probably reword the question to work around these problems, along the lines of:
What are the characteristics of systems that scale up, rather than out?
And have a perfectly valid, on-topic question that wouldn't have any problem getting reopened.