As for why they didn’t go after anyone else, I don’t know. It could be laziness, it could be that others did a better job covering their tracks and the government doesn’t think they’ll get a conviction, or it could be those individuals are in countries that won’t extradite / prosecute even if there is evidence.
As I said, I think the sentences are bullshit, but I’m not sure Bowser got singled out. He just happened to be in a country that actually extradited him.