It's because he's the antithesis of German values. In Germany, children are taught that hard work is the only option, and laziness and mediocracy are shameful. Rainer doesn't care that he's a mediocre slob who doesn't contribute in any way. Worse yet, he's not ashamed. He gleefully shares what a useless piece of shit he is.
It's not that. It's that he is provoking everyone. He called construction workers too lazy and too dumb to get a better job. He provoked so many people; music fans (metal, punk, hip hop), antifascists (he is racist af), actually, he provoked any group of people except nazis (which he claims to know 20 of and can call immediately and they will fuck up his haters). If he would just show his boring and lazy ass, nobody would care. But he provokes people, is incredibly rude, racist, sexist, dumb and ignorant. He told people a few times to come by so he can punch them. And so they came.
That he blew up so much is because he is so incredibly dumb and people first made fun of him online. Which he didn't understand and thought everyone loves him. He started doing vlogs of his house and how he lives when he had 200 subs on YouTube. Normally people start with that shit when they reach an actual milestone like 500k subs. Or more.
There are so many people similar to Rainer in the German region (Switzerland, Germany, Austria) but they mostly stick to themselves or withdraw themselves after confrontation or whatever. But Rainer is not capable of reflecting or admitting anything. When he does is, he pretends to reflect and see his mistakes. But he actually doesn't see what he did and does wrong. He just vaguely says "I did shit wrong too. Like I shouldn't have said my address." That's all he says to that topic. His holocaust comment or the Jews statement, those weren't mistakes in his eyes.
Rainer keeps provoking people with what he says (ignorant, dumb, sexist, racist statements) and does (flash his penis in front of a kid, sending nudes to teenagers, mistreating/abusing animals,...). It is far beyond "trying to make fun of the internet clown", which it was at the start. He pissed off too many people and visiting his place is like going into the zoo - personally I don't agree on visiting him but nobody respects him and his privacy anymore. And they don't care about a fine, they want to make him mad, even if it's just chilling in front of his house. That sets him off already.
Tl;dr: it's not that he is the antithesis of German values. He provokes people and tells them to come by so he can punch them.