By the way, if match-fixing is well organized, it is very difficult to detect. Of course, it is more common in the lower leagues, but on the other hand, maybe it is simply more often detected in the lower leagues and poor countries? Sometimes it is so poorly hidden that it becomes noticeable almost immediately. And we only know about match-fixing from those cases that were easily detected. And those fixed matches that remained undetected contributed to the funding of the criminal industry. Do we have the tools to detect well-organized match-fixing? I doubt it.
I agree. In lower leagues and tournaments, it is done more often and there can be several reasons for that. The first is probably that the cost is lower for those fixing the matches because players or even the officials of lower leagues would accept lower amounts, which means that if a league is available for betting but is from a lower stage, the fixers can do their thing and then bet on the games to win huge amounts without having to spend a lot of money on it.
On bigger stages, they will need to spend more money, and that increases the risk of losing all that money in case the fixing gets caught. You are also right that they probably also do it on bigger stages but in more sophisticated ways so that it is not easily noticeable because they are getting smarter over time. They can cheat an organization and get away with it without anyone doubting.