At last, an authoritative explanation:

Trump lacks certain qualities which the British traditionally esteem.  For instance, he has no class, no charm, no coolness, no credibility, no compassion, no wit, no warmth, no wisdom, no subtlety, no sensitivity, no self-awareness, no humility, no honour and no grace….

Plus, we like a laugh. And while Trump may be laughable, he has never once said anything wry, witty or even faintly amusing – not once, ever.  I don’t say that rhetorically, I mean it quite literally: not once, not ever. And that fact is particularly disturbing to the British sensibility – for us, to lack humour is almost inhuman.

But with Trump, it’s a fact. He doesn’t even seem to understand what a joke is – his idea of a joke is a crass comment, an illiterate insult, a casual act of cruelty….

And scarily, he doesn’t just talk in crude, witless insults – he actually thinks in them. His mind is a simple bot-like algorithm of petty prejudices and knee-jerk nastiness.

There is never any under-layer of irony, complexity, nuance or depth. It’s all surface.

(Thanks to Clayton Agree for the pointer.)