I think it's because for a certain type of British person who also dislikes London, Paris is London, only with a different language that kind of British person is unlikely to speak. So, basically, the hated capital, with all the ordinary brusqueness, litter, infrastructural problems, petty crime etc of any busy capital city, but with FOREIGNNESS! and LANGUAGE ISSUES! into the bargain.
And this romanticisation of it, which is frankly silly ( yes, in many ways, it's a beautiful city, and some of the most romantic moments of my life have happened there (I have very fond memories of the Square du Vert-Galant), but it also just a city, full of people who live there, work there, do their dry-cleaning there and buy their toilet paper and UHT milk there) -- if people are expecting Disney, they will be disappointed.
I'm not British, speak formerly good, now rusty, French and lived in Paris for a few years as a graduate student, aeons ago, and I've never found it particularly rude/unkempt etc compared to London, where I lived for ten years.
For instance, I see people complaining about the rudeness of shop assistants, but by French norms, a customer is being very rude not to say 'Bon jour' when they enter. You adjust to the host country's norms.
I had to make a choice about whether to accept a job in Paris or in London, and I chose London, ultimately -- which I think was the right choice. My issue with Paris is how conservative it is in many ways, but I'm still fond of it, and spend a fair bit of time there.