Probably because to use the term "corruption" in the media/press is quite a direct accusation and potentially will be challenged as libel (rightly or wrongly).

"Sleaze" is less tangible and probably harder to challenge, but carries similar weight. I think sleaze also covers non-corruption activities too - general face-saving in light of embarrassing-but-legal things like quirky sexual activities, substance misuse (where legal), questionable (not not "bad") moral decisions, questionable (but not illegal) business decisions etc etc