I work in the field of human trafficking.
The reasons are multiple and no one size fits all. The question I never understood (before doing this work) is why not claim in Italy, Luxembourg,Belgium, or France.. ?
I have a few answers. From Migrants themselves.
Language. English is the most widely spoken second language in Europe. The ability to communicate even basically cannot be overlooked.
To many migrants, UK is still the 'Mothership' for want of a better word.. for former colonies. Therefore many many have either family or at least one friend here.
We tend to stick to international laws. To a fault. The asylum system is fucked and the immigration system more so... if you stick it out.. get married quick have kids double quick you can claim as right to family life ' which is OUTSIDE the immigration rules.
Compared with many European countries we are actually racially tolerant.
Last but not least .. said to me two weeks ago at a processing centre .. 'UK very good for people who make business.
I'll add one of my own though as I work at the more unpleasant end of immigration..
No ID cards . It's a real thing.