As far as I can see only George Bush and Obama had formal state visits hosted by Elizabeth II. So I suppose the question is why is Trump so exceptional to warrant such a visit?
I am sure he does. With the UK in such a weak negotiating position, there is nothing they couldn't force us to take. Chlorinated chicken, reduction in safety standards, access to internal NHS etc.
I think many people appreciate the safe end to end rearing of chicken vice the US model where hygiene and welfare considerations are secondary because the faeces encrusted meat is just dipped in chlorine at the end of the process.