I used to live in Thailand where locals getting a tourist visa to visit Schengen countries is fairly straightforward but locals getting a tourist visa to visit the UK was practically impossible. I cannot see a reason for this as, so far as I'm aware, there is no wish on the part of most Thais to move to the UK, especially not illegally by entering via a tourist visa. My conclusion therefore is that the government dept responsible is run by bigoted, racist, fückin arseholes!
In my three years over there, I met dozens of Thais who applied for a UK tourist visa; many were in stable relationships with either Brits or other EU nationals (this was before Britain left the EU) and whilst none that I met were refused a Schengen visa if they applied for one, equally none were granted a UK visa; not one single one!