Fetterman immediately was endorsed by two large unions (steel workers and food service) representing 80000 Pennsylvanians. Within just two days of announcing his candidacy he has received 37000 small dollar donations from all 67 counties in Pennsylvania and all 50 states.
Fetterman was endorsed by Bernie Sanders in all of his election cycles and was one of the only people elected to high office in Pennsylvania to endorse Sanders for president in 2020.
It's all of the above. Organizing people takes real effort and skill even if they're primed to hear your message by the failures of capitalism. Consider how many people hate the symptoms of capitalism but don't spontaneously become leftists or spontaneously form leftist organizations.
As for why it's not smart for Bernie to just openly break campaign finance laws: First, capitalist reaction to someone who generally serves capital (Trump) will not be the same as capitalist reaction to someone who challenges it (Bernie). Look no further than how laws are enforced against BLM protesters vs. how they're enforced against right-wing cranks. Second, a lot of the people Bernie (and his successors) appeal to don't want someone who wipes their ass with the law, even if it's for a good cause. Trump's base doesn't care when he does this stuff because they already know he's a piece of shit; Bernie might not get the same pass because he comes off as a generally decent person.