Why do some towns/cities have a bad reputation?

I’m relatively new to the UK, and one thing I’ve noticed is that certain towns seem to have a really bad reputation, sometimes almost as a running joke. For example, I recently heard a colleague groaning about having to travel to Milton Keynes for work. I’ve been there a couple of times and didn’t think it was bad at all. If anything, I actually liked parts of it. The roads are well planned, the cycle paths are genuinely usable (most cities here have decent roads but uneven and broken sides/cycle paths), and lots of roundabouts. I note that it lacks historic buildings, old town centres, and traditional character that other places may have. But the way people talk about it, I expected it to be absolutely dreadful and it really wasn’t. Similar experience with Swindon. I’ve driven through it a few times and while I wouldn’t call it charming, it didn’t seem anywhere near as terrible as its reputation suggests. So I’m curious - is there some historic or cultural reason why places like Milton Keynes and Swindon has a bad rep? Also, are there any other UK towns/cities that people love to hate, but which you think are actually unfairly judged? (And yes, before anyone says it, I know some places seem to fully live up to their reputation.)