I get that, the generic, almost unchanged for two decades shape, way too familiar interior and switchgear, etc etc. But let's look closer - out of the "good bunch"... - so let's say we have Jaguar, Merc, Audi and BMW, right? Jaguar is one foot in the grave for many years, British made, odd trim choices, handful of dealers in network, not really a competition. Merc. Had really bad decade, reliability issues, off the wall service pricing (B service costs to the tune of £500, and it's just a glorified filter and oil swap, some dealers charge £150 per hour and diagnostics fee just to look at your car etc etc). So that leaves you with beemer and audi. Suddenly choosing Audi out of good bunch is simply choosing "second contender", "the other best choice". It's choosing AEG over Miele. Panasonic over Sony. PRS over Gibson.
Additionally for many people Audi is natural progression within brand. Anecdotal brickies mate will have a Polo, promoted to brickie he will upgrade to Golf, as a foreman he would have Tuareg, once proprietor, he will probably reach for Audi. He knows his architect had RS and was very happy with it. The mundane familiarity of "it's just like Seat/Skoda with Audi badge" actually works for it, rather than against it.
And then, uncomfortably, let's not forget that premium brands attract also posers. And without dancing around subject, for your typical yuppie, canary wharf underground parking junior poser, Audi is the least ethnically tinted choice.