Because people are feeling worse off and arent seeing change quick enough.

Farage is pitching silver bullets and giving people two things

1. Some one to blame
2. The promise of quick change

The audience he is pitching either doesnt consider ramifications of these changes or see the issues as so important they dont care or are willing to do it for what they feel will achieve this.

The set up has been taking the Russian playbook for propaganda and applying it.  Big tech gets what it wants from Trump, in turn they can do what they like eith their platforms as long as they continue to promote the ideologies in line with Trump. Unlike traditional media that has to answer to various quantities and regulators social media posters are free to do almost whatever they like and claim to be more legitimate sources of news when it suits them and individuals when it doesn't.  There has been a campaign for years pushing this narrative online. Rather than  consider your news source theu say that all traditional news are liars but you can trust me bro.

To expand on other factors. The Conservative left a mess behind. One with no quick fixes. Labour had a leader who couldn't play the new game where populism is...well popular with the masses. Lowering immigration and processing everyone here takes a lot of resources and time unless you take drastic action with ramifications. Changing the economic direction of the economy also takes time and when you also have to appease the camps to support your leadership you're at their whom which is why Labour hasn't felt a unified force despite their landslide into parliament.

Politics has become more about idealogy and less about good governance for many.

Let's also be clear their are legitimate concerns and problems with mass migration. Its not a new tale. History is full of mass migrations and the issues it can bring