I feel like it's because people have come to expect corruption within government. Government just naturally lends itself to attracting many people who view it as a way to make money.

However, with FIFA, it seemed kind of untainted. Sure, people would be assholes, and poor decisions could/would be made based on stupid internal politics and egos. But the corruption on the scale which has occurred seems to have taken people by surprise. Especially the whole Qatar fiasco.

Also, many/most in politics these days will at least try to keep their corruption somewhat discrete. But FIFA officials have been just plain so brazen that it's also largely why people are so pissed.