What gives "diner" coffee its distinctive taste? At least in the US (and probably elsewhere), late-night diners are a popular place to get a cheap cup of joe. In my experience, diner coffee has a taste that's generally not found elsewhere (most people call it "bad", but that's not what this question is about).

I'm wondering what causes this taste, and how they all get such similar tasting coffee? I assume the brewing process has something to do with it (those big commercial drip brewers which they all seem to use), but I've been to other places that used those with various different roasts and they all tasted different.

So what else makes diner coffee taste like diner coffee? Examples of the kinds of places that serve this include chains like Waffle House, but also individual / locally owned diners.
I think it is a combination of old stale grounds sitting on a burner for long hours. The places you mention are generally open 24/7 or something close to it and have people ordering coffee at all times of day, so it's all made ahead of time and kept warm all day.

The cheapest coffee suppliers usually mix beans from all over the place and they all store them way past any freshness date. Diners run on small budgets and generally buy the cheapest stuff, thus they all have the same "flavor".

That combination is not just a diner thing, I would also say it is the same as cheap gas station coffee in the US.

I would knock it more if it didn't keep me awake behind the wheel more than a few times.