Underrated performance by Tom Cruise

He probably would have easily earned a best supporting nod since Foxx was the lead. But Cruise gave himself top billing as the lead.

He shows some incredible acting chops in Born on the 4th of July as well.

Yeah, that's the thing: a lot rides on who else is nominated. Just when you think it's impossible that Chinatown lost the Gold, you realise it was up against The Godfather Part II. The Conversation was also nominated that year, so you had three amazing movies that, in many other years, could easily have beaten the pants off of any other competition. But because it was that year, two of them had to lose to the other.

It's also hard to compare performances. For example, Johnny Depp was nominated for Jack Sparrow in the same year Bill Murray was nominated for Bob Harris (Lost in Translation, both losing to Sean Penn's performance in Mystic River. I haven't actually seen Mystic River, but I don't know how to stack Jack Sparrow up against Bob Harris. They are such different performances with different goals. Jack is also clearly the supporting character...

Which is what you're getting at by pointing out that Cruise should have been in the supporting category. But studios play loose with the categories to hedge bets and try to get wins. It's the same as voters tossing political wins as kind of surrogate "career achievement awards." Somebody lost three times in a row (usually due to the same kind of situation as Godfather II v. Chinatown) and so their "owed" the next won and beat out somebody who actually should have won that year.