The simple answer to that is that Tesla do an amazing job in spite of not advertising of convincing the general public that they are at the apex of autonomous driving, and tech generally.
A lot of that I guess has to do with the fact that Elon gets endless column inches written about his promises, completely credulously. Various news sites seeking clicks leverage him and in so doing further cement the myth that they are at the cutting edge.
I would probably argue that Elon's purchase of Twitter has gone some way to diminishing this, though. Since pretty much every decision he has made with that business has been transparently terrible, I am finding more and more of these same news sites reporting on him in a far less favourable light. I don't think he can bullshit his way through saying "FSD is going to be here this year", "Robotaxis will be in your living room for $25k by 2024", etc anymore.
In terms of the UK - autopilot, FSD, etc has basically stood still for at least as long as I've owned my car (Q1 2020), while other manufacturers are making significant progress or surpassing them (Mercedes level 3 validation springs to mind). For all the overtures about Tesla's constantly being upgraded in software terms, it's all very surface level stuff, and it is like a salve to distract you from the fact that basic things like intelligent lighting, wipers, even parking sensors that work, etc is conspicuously absent on this "car for the techie minded", and the USP of autonomous driving etc is at a standstill.
Taken as a whole this ultimately means that the public perception of where Tesla is with their cars, and autonomous driving, is very different to the reality experienced by any owner. I've said it several times before by my work colleagues think my car can drive me home from a club while I'm asleep or drunk in the passenger seat.
I can imagine many Tesla customers being once bitten, twice shy when it comes to looking at replacing their car, and being a one Tesla owner as a result.