I'm not sure they choose not to join UKP.
Perhaps more a case of not being aware of the site?
I've been an active punter since the early 1990's & at some point, became aware of
another punting site where reviews were left but, IMHO, that site was much more biased towards service providers.
To this day, I cannot remember how I came across UKP but I've been here since 2012 & I think it's a much better site than the 'other' one was.
When I first joined, I was taken aback by a few of the (let's say) more robust members (several names spring to mind, no longer here, thank heavens) & I wasn't sure I would last the course. Although I found UKP very useful indeed,
I did not like it as an entity.Nowadays, again, IMHO, it's a far better site than it once was, much more fair, less quarrelsome amd a great deal fewer shit-stirrers
Although I'm not punting anywhere near as much as I used to when the money was 'easy come, easy go' - I'm still on this site every day for a short while and, it's the only site I use.
Long may it continue
FB