Because he's a relatively skilled speaker. Like many others in the 1980s, he was literally taught the art of persuasive speaking - or rhetoric as it's properly known. So was Johnson as it happens. Trump's a monkey in comparison to these guys.
And because Farage learned from Enoch Powell, a successful Cambridge academic with a background in classics. The thing that Powell understood very well was that if you wanted the people to vote for you, then you must appear like you're on their side. There are various ways of doing this, for example, in Powell's case, for his River of Blood speech, he simply made up the housewives and concerned folk who he was quoting. Neither they, or the problems they were talking about, actually existed. But it didn't matter because it SEEMED real to people.
Farage is cut from the same cloth. His background also involves fiction - commodity trading and futures. He knows how to work a market, how to operate the levers and pulleys. And in the same way that people believe money is somehow real (when actually it's just numbers or bits of plasticised paper in their wallet), they believe what he's saying has some sort of basis in fact. It doesn't. He's a kind of one man casino. But you know what they say, the house always wins.
There's an old Latin tag: cui prodest? It means 'Who stands to gain?' Farage will know this in its original Latin. Powell studied it and understood it intimately. The average bod in the street has no idea that they're just the pawns in this game, the future casualties, the cannon fodder. They're clueless. They have neither the education or the skills to work out what's happening. And so like turkeys they'll vote for Christmas and then wonder why their teenage children / grandchildren are being called up to die in ditches.
Those who do not understand the lessons of history are doomed to repeat it, eh.